Identification of compromised components in a medical system

ABSTRACT

In some examples of selecting a target therapy delivery site for treating a patient condition, a relatively high frequency electrical stimulation signal is delivered to at least two areas within a first region (e.g., an anterior nucleus of the thalamus) of a brain of a patient, and changes in brain activity (e.g., as indicated by bioelectrical brain signals) within a second region (e.g., a hippocampus) of the brain of the patient in response to the delivered stimulation are determined. The target therapy delivery site, an electrode combination, or both, may be selected based on the changes in brain activity.

TECHNICAL FIELD

The disclosure relates to medical devices and, more particularly, tomedical devices for therapeutic brain stimulation.

BACKGROUND

Implantable medical devices, such as electrical stimulators ortherapeutic agent delivery devices, may be used in different therapeuticapplications, such as deep brain stimulation (DBS) or the delivery ofpharmaceutical agent to a target tissue site within a patient. A medicaldevice may be used to deliver therapy to a patient to treat a variety ofsymptoms or patient conditions such as chronic pain, tremor, Parkinson'sdisease, other types of movement disorders, seizure disorders (e.g.,epilepsy), obesity or mood disorders. In some therapy systems, anexternal or implantable electrical stimulator delivers electricaltherapy to a target tissue site within a patient with the aid of one ormore implanted electrodes, which may be deployed by medical leads or ona housing of the stimulator. In addition to, or instead of, electricalstimulation therapy, a medical device may deliver a therapeutic agent toa target tissue site within a patient with the aid of one or more fluiddelivery elements, such as a catheter or a therapeutic agent elutingpatch.

SUMMARY

In general, the disclosure relates to devices, systems, and methods foridentifying a component failure in an implanted medical device system,such as a system implanted to deliver deep brain stimulation (DBS)therapy. One way to detect a failure is by using acutely-measuredimpedance data. Such data may provide impedance information regarding animplanted DBS system that may include an implantable device, a leadextension, and a lead. Such impedance information may provideinformation on how the system components are interacting with the tissueand/or with one another. For instance, the data may indicate whether acomponent has experienced a short of an open circuit. Not allout-of-range (e.g., unusually low or high) impedance values areassociated with verifiable hardware complications. Sometimes high or lowimpedance values may be the result of unusually high or low tissueimpedance values, for instance. Conversely, sometimes verifiablehardware problems may exist that do not result in acutely-measuredout-of-range impedance values.

For instance, in the case of an intermittent open or short circuit, anacutely-measured impedance value may be in-range most of the time. Theopen or short circuit may only manifest itself when a system componentundergoes a certain type of movement or is flexed into a certain shapeor position. Such a fault may be difficult to detect when only acutemeasurements are used. In this latter case, a loss of DBS therapybenefit may result (at least intermittently) from this type ofundetected electrical shunt or short.

In accordance with the foregoing, the current disclosure providestechniques to identify and diagnose DBS system connection integrityissues for gauging the impact on therapy and suggest potentialsolutions. In one embodiment, a system is disclosed comprising a sensorconfigured to sense a first signal at a first location of an anatomy ofa patient, one or more processors configured to associate a portion ofthe first signal with a second signal introduced at a second location ofthe anatomy of the patient and to determine whether a fault exists inthe system based on one or more characteristics of the associatedportion of the first signal. As another example, a method of detecting afault in an implantable medical device system is disclosed comprisingsensing a first signal via an electrode at a first location of ananatomy of a patient, associating a portion of the first signal with asecond signal introduced at a second location of the anatomy of thepatient, and determining whether a fault exists in the system based onone or more characteristics of the associated portion of the firstsignal, wherein one or more of associating a portion of the first signalwith a second signal introduced at a second location in the anatomy ofthe patient and determining whether a fault exists in the system areperformed by one or more processors.

The details of one or more examples of the disclosure are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages will be apparent from the description anddrawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an example deep brainstimulation (DBS) system configured to deliver an example electricalstimulation therapy to a target tissue site within a brain of a patient.

FIG. 2 is a conceptual diagram illustrating an example therapy systemconfigured to deliver a therapeutic agent to a tissue site within abrain of a patient.

FIG. 3 is functional block diagram illustrating components of an examplemedical device.

FIG. 4 is a functional block diagram illustrating components of anexample medical device programmer.

FIG. 5 is a conceptual diagram illustrating example regions of a brainof a patient, and, in particular, regions of the brain included in theCircuit of Papez.

FIG. 6 is a flow diagram illustrating an example technique for usingsensed signals to determine whether a potential fault may exist withinthe system.

FIG. 7 is a flow diagram illustrating sensing an evoked potential at afirst location when stimulation is delivered to a second location.

FIG. 8 is a flow diagram illustrating used of a baseline signal inanalyzing whether a system fault has occurred within the system.

FIGS. 9A and 9B are examples of waveforms representing signals sensed inthe brain of a patient during a fault condition and during a non-faultcondition, respectively.

FIG. 10A illustrates multiple plots of impedance values measured betweenvarious electrode pairs of an implantable 3387 model lead, which iscommercially available from Medtronic, Inc., that has been implanted inan ovine subject.

FIG. 10B is a graph of impedance values between various electrode pairsof a system comprising a 3389 model lead commercially available fromMedtronic, Inc. which has been implanted in an ovine subject.

FIG. 10C is a graph of multiple plots of impedance values measuredbetween various electrode pairs of an implantable 3389 model leadimplanted in an ovine subject.

FIG. 10D is another graph of plots of impedance values measured betweenvarious electrode pairs of an implantable 3389 model lead implanted inan ovine subject.

FIG. 11A is a conceptual diagram illustrating the frequency content ofan LFP signal over time in a system free of a fault.

FIG. 11B is a conceptual illustrating the frequency content of an LFPsignal over time in a system that includes an intermittent fault in thelead sensing an LFP signal in the STN.

FIG. 12 is a flow diagram illustrating a method of diagnosing a fault bydetecting an artifact in the signal.

FIG. 13 is a signal diagram of two LFP recordings obtained from the ANand HC of an ovine subject as may be used to determine whether a faultexists within the system.

FIG. 14 is a flow diagram illustrating therapy titration performed todetermine whether a change in therapy efficacy may have occurred.

FIG. 15 is a flow diagram illustrating an example method according tothe current disclosure.

FIG. 16 is an example of a dashboard screen that may be provided toreport information used to diagnose potential faults as describedherein.

FIG. 17 is another example illustration of a dashboard screen that mayprovide information to aid in fault diagnosis.

FIG. 18 is an illustration of another dashboard screen that providesinformation to aid in diagnosing system faults.

FIG. 19 is another example illustration of a dashboard screen that maybe provided to help a user to determine if a system has a fault.

DETAILED DESCRIPTION

The present disclosure relates to techniques for detecting and analyzingindicators of compromised DBS system components such as leads and leadextensions. Compromised components may involve those associated withshort circuits or electrical shunts. Such faults may be indicated basedon signals sensed from one or more DBS leads. Specifically, impedancevalues, evoked potential measurements (EPs), and identification ofmovement or ECG artifacts may be detected in the sensed brain signals.One or more of these sensed brain signals may be used to assess afailure of the system component(s). In one example, a DBS lead or leadextension is determined to be functioning or non-functioning based on anaggregate outcome of a plurality of these measures. According to thisapproach, failures may be detected that would otherwise go unidentifiedif failure analysis were based solely on acute impedance measurements.FIG. 1 is a conceptual diagram illustrating an example therapy system 10that is configured to deliver therapy to patient 12 to manage a disorderof patient 12. In some examples, therapy system 10 may deliver therapyto patient 12 to manage a seizure disorder (e.g., epilepsy) of patient12, which is characterized by the occurrence of seizures. Therapy system10 may be used to manage the seizure disorder of patient 12 bypreventing the onset of seizures, minimizing the severity of seizures,shortening the duration of seizures, minimizing the frequency ofseizures, and the like. Patient 12 ordinarily will be a human patient.In some cases, however, therapy system 10 may be applied to othermammalian or non-mammalian non-human patients. While examples of thedisclosure are described in some cases with regard to management ofseizure disorders, in other examples, therapy system 10 may also providetherapy to manage symptoms of other patient conditions, such as, but notlimited to, Alzheimer's disease, psychological disorders, mooddisorders, movement disorders like Parkinson's disease or otherneurogenerative impairment.

Therapy system 10 includes medical device programmer 14, implantablemedical device (IMD) 16, lead extension 18, and one or more leads 20Aand 20B (collectively “leads 20) with respective sets of electrodes 24,26. IMD 16 includes a stimulation generator that is configured togenerate and deliver electrical stimulation therapy to one or moreregions of brain 28 of patient 12 via a subset of electrodes 24, 26 ofleads 20A and 20B, respectively. In the example shown in FIG. 1, therapysystem 10 may be referred to as a deep brain stimulation (DBS) systembecause IMD 16 provides electrical stimulation therapy directly totissue within brain 28, e.g., a tissue site under the dura mater ofbrain 28. In other examples, leads 20 may be positioned to delivertherapy to a surface of brain 28 (e.g., the cortical surface of brain28). In some examples, delivery of stimulation to one or more regions ofbrain 28, such as an anterior nucleus of the thalamus (also referred toherein as the “anterior nucleus,” “anterior thalamic nucleus” or “AN”),the subthalamic nucleus (STN) or cortex of brain 28, may provide aneffective treatment to manage a seizure or other disorder of patient 12.

Electrical stimulation generated from the stimulation generator (shownin FIG. 3) of IMD 16 may help prevent the onset of events associatedwith the patient's disorder or mitigate symptoms of the disorder. Forexample, electrical stimulation therapy delivered by IMD 16 to a targettherapy delivery site within brain 28 may help minimize the occurrenceof seizures or minimize the duration, severity or frequency of seizuresif patient 12 has a seizure disorder.

IMD 16 may deliver electrical stimulation therapy to brain 28 of patient12 according to one or more stimulation therapy programs. A therapyprogram defines one or more electrical stimulation parameter values fortherapy generated and delivered from IMD 16 to brain 28 of patient 12.In examples in which IMD 16 delivers electrical stimulation in the formof electrical pulses, for example, the stimulation therapy may becharacterized by selected pulse parameters, such as pulse amplitude,pulse rate, and pulse width. In addition, if different electrodes areavailable for delivery of stimulation, the therapy program may includeone or more electrode combinations, which can include selectedelectrodes (e.g., selected from electrodes 24, 26) and their respectivepolarities. The exact therapy parameter values of the stimulationtherapy that helps prevent or mitigate seizures, such as the amplitudeor magnitude (electrical current or voltage) of the stimulation signals,the duration of each signal (e.g., in the case of stimulation pulses, apulse width or duty cycle), the waveform of the stimuli (e.g.,rectangular, sinusoidal or ramped signals), the frequency of thesignals, and the like, may be specific for the particular targetstimulation site (e.g., the area of the brain) involved as well as theparticular patient and patient condition. While stimulation pulses areprimarily described herein, stimulation signals may be of any form, suchas continuous-time signals (e.g., sine waves) or the like.

In addition to delivering stimulation therapy to manage a disorder ofpatient 12, therapy system 10 is configured to monitor one or morebioelectrical brain signals of patient 12. For example, IMD 16 mayinclude a sensing module that senses bioelectrical brain signals withinone or more regions of brain 28. In some examples, the sensing module ofIMD 16 may receive the bioelectrical signals from electrodes 24, 26 orother electrodes positioned to monitored brain signals of patient 12. Inthe example shown in FIG. 1, the signals generated by electrodes 24, 26are conducted to the sensing module within IMD 16 via conductors withinthe respective lead 20A, 20B. Electrodes 24, 26 may also be used todeliver electrical stimulation from the therapy module to target siteswithin brain 28 as well as sense brain signals within brain 28. In someexamples, the sensing module of IMD 16 may sense bioelectrical brainsignals via one or more of the electrodes 24, 26 that are also used todeliver electrical stimulation to brain 28. In other examples, one ormore of electrodes 24, 26 may be dedicated to sensing bioelectricalbrain signals while one or more different electrodes 24, 26 may bededicated to delivering electrical stimulation.

In some examples, a sensing module of IMD 16 may sense one or more firstsignals at a first location in the patient's anatomy (e.g., the brain)that contain some portion that may be associated with a second signalintroduced into the patient's anatomy at a second location (e.g., asecond location in the brain). That second signal may be, for instance,a stimulation signal delivered to the second location. This stimulationsignal that is delivered to the second location may result in an evokedresponse (also referred to herein as an evoked potential) which may bedetected within the one or more first signals.

As another example, the second signal introduced into the patient'sanatomy may be associated with movement of a portion of the patient'sbody that results in motion artifacts being introduced at a secondlocation. These motion artifacts may be evident in the one or more firstsignals sensed at the first location in the patient's anatomy, which isdifferent from the second location. As yet another example, the secondsignal may be an artifact resulting from any other signal introducedinto the body, such as an electrocardiogram (ECG) artifact introduced byactivity (e.g., beating) of the patient's heart. A portion of the sensedsignal at the first location may correspond with one or morecharacteristics of this signal introduced at this second location(within the patient's chest).

As discussed above, a portion of the sensed signal may be associatedwith this second signal that is introduced at the second location. Inother words, a time correlation may exist or be determined between theportion of the sensed signal and the signal at the second location. Forinstance, the portion of the sensed signal may occur contemporaneouslywith introduction of the second signal at the second location. Theportion of the sensed signal may be known to occur at the same time as,or within an expected time delay after, the second signal is introducedat this second location. Thus, this portion of the sensed signal has atemporal relationship with the second signal that may be used in somecases to determine whether there is a potential fault in one or moresystem components or interconnection between system components as willbe discussed further below.

In some examples, bioelectrical signals sensed by IMD 16 within brain 28of patient 12 or a separate sensing device implanted or external topatient 12 may be used to detect a compromised component orinterconnection of the system. The compromised component can be, forexample, a lead or lead extension that has a short or a leakage pathway.A leakage pathway may result when the insulation in the lead or leadextension has damage or imperfection that results in incomplete sealingand/or insulation of the lead conductors. A leakage pathway may alsoresult when connections between the lead extension and the connectorblock 30 of the IMD 16 or the lead and the lead extension are notsealing properly such that fluid ingress in the connector block causes alow impedance pathway that compromises system operation. Similarly, afaulty connection between a lead 20 and the connector bock 30 of the IMD16 (in those situations wherein a lead extension 18 is not used) mayallow fluid ingress, again resulting in leakage paths that compromisesignal transmission. Such faults can be detected by sensingbioelectrical signals according to techniques of the current disclosure.

Various combinations of electrodes may be used by IMD 16 for sensingpurposes and for delivering stimulation to a patient. Moreover, sensingand delivery of stimulation may be performed at various locations in apatient's anatomy. For instance, IMD 16 may monitor brain signals anddeliver electrical stimulation at the same region of brain 28 or atdifferent regions of brain 28. In some examples, the electrodes used tosense bioelectrical brain signals may be located on the same lead usedto deliver electrical stimulation, while in other examples, theelectrodes used to sense bioelectrical brain signals may be located on adifferent lead than the electrodes used to deliver electricalstimulation. In some examples, a bioelectrical brain signal of patient12 may be monitored with external electrodes, e.g., scalp electrodespositioned over a temporal lobe of brain 28. Moreover, in some examples,the sensing module that senses bioelectrical brain signals of brain 28(e.g., the sensing module that generates an electrical signal indicativeof the activity within brain 28) is in a physically separate housingfrom outer housing 34 of IMD 16. However, in the example shown in FIG. 1and the examples primarily referred to herein for ease of description,the sensing module and therapy module of IMD 16 are enclosed within acommon outer housing 34.

As previously stated, bioelectrical brain signals sensed by IMD 16 mayreflect potential faults in system components. Examples of the sensedbioelectrical brain signals include, but are not limited to,electroencephalogram (EEG) signals, electrocorticogram (ECoG) signals,local field potentials (LFP) sensed from within one or more regions of apatient's brain and/or action potentials from single cells within brain28.

In the example shown in FIG. 1, IMD 16 may be implanted within asubcutaneous pocket above the clavicle of patient 12. In other examples,IMD 16 may be implanted within other regions of patient 12, such as asubcutaneous pocket in the abdomen or buttocks of patient 12 orproximate the cranium of patient 12. Implanted lead extension 18 iscoupled to IMD 16 via connector block 30 (also referred to as a header),which may include, for example, electrical contacts that electricallycouple to respective electrical contacts on lead extension 18. Theelectrical contacts electrically couple the electrodes 24, 26 carried byleads 20 to IMD 16. Lead extension 18 traverses from the implant site ofIMD 16 within a chest cavity of patient 12, along the neck of patient 12and through the cranium of patient 12 to access brain 28. IMD 16 isconstructed of a biocompatible material that resists corrosion anddegradation from bodily fluids. IMD 16 can comprise a hermetic outerhousing 34, which substantially encloses components, such as aprocessor, therapy module, and memory. In the example shown in FIG. 1,leads 20 are implanted within the right and left hemispheres,respectively, of brain 28 in order deliver electrical stimulation to oneor more regions of brain 28, which may be selected based on manyfactors, such as the type of patient condition for which therapy system10 is implemented to manage. Other implant sites for leads 20 and IMD 16are contemplated. For example, IMD 16 may be implanted on or withincranium 32 or leads 20 may be implanted within the same hemisphere orIMD 16 may be coupled to a single lead that is implanted in one or bothhemispheres of brain 28.

Leads 20 may be positioned to deliver electrical stimulation to one ormore target tissue sites within brain 28 to manage patient symptomsassociated with a condition of patient 12. Leads 20 may be implanted toposition electrodes 24, 26 at a target implantation site within brain 28via any suitable technique, such as through respective burr holes in askull of patient 12 or through a common burr hole in the cranium 32.Leads 20 may be placed at any location within brain 28 such thatelectrodes 24, 26 are capable of providing electrical stimulation to oneor more target therapy delivery sites within brain 28 during treatment.

Different neurological or psychiatric disorders may be associated withactivity in one or more of regions of brain 28, which may differ betweenpatients. For example, in the case of a seizure disorder or Alzheimer'sdisease, leads 20 may be implanted to deliver electrical stimulation toregions within the Circuit of Papez, such as, e.g., the anterior nucleus(AN) of the thalamus, the internal capsule, the cingulate, the fornix,the mammillary bodies, the mammillothalamic tract (mammillothalamicfasciculus), and/or hippocampus. Regions of brain 28 may be functionallyconnected to one another via neurological pathways such that activitywithin one region of brain 28 may affect activity within another regionof brain 28. For example, electrical stimulation delivered by IMD 16 toa particular region of brain 28 may influence brain signals in one ormore other regions of brain 28. In some examples, brain activity can beindicated by a signal characteristic (e.g., an amplitude, frequency,and/or frequency domain characteristic) of a bioelectrical brain signal.As an example, the signal characteristic of a bioelectrical brain signalsensed within a particular region of brain 28 may change as the brainactivity in the region changes.

One example of functionally connected regions of brain 28 includes theCircuit of Papez (described below with respect to FIG. 5). Electricalstimulation delivered from IMD 16 to a particular region of the Circuitof Papez may influence brain signals in one or more other regions of theCircuit of Papez. In other words, a brain signal sensed at one locationin the Circuit of Papez may contain one or more characteristics that areindication of a response to stimulation in another different location inthis circuit. In some examples, the one or more characteristics comprisea waveform that is indicative of an evoked response (i.e., an evokedpotential) that occurred within the brain because of stimulation inanother different location in the circuit.

As described in further detail below, in some examples, a potentialsystem fault may be detected based on one or more characteristics of asignal sensed in a first region (first location) of the brain as theresult of electrical stimulation delivered to a second region of brain28 that is different than the first region, but functionally connectedto the first region. The electrical stimulation delivered to the secondregion of the brain 28 may be relatively low frequency stimulationwhich, in one example, may have a frequency ranging between 1 and 20 Hz,and in more particular examples between 2 and 10 Hz. Delivery of lowfrequency stimulation at one location in the anatomy (e.g., at a secondregion of the brain) may be associated with at least a portion of abiological signal sensed in the first location in the anatomy (e.g., afirst region or location of the brain). For instance, a portion of thesensed biological signal that was sensed during delivery of thisstimulation and/or within some predetermined time delay thereafter maybe the portion that is associated with this stimulation signal, becauseit is the portion the sensed biological signal that has the possibilityof being affected by this stimulation. This portion of the sensedbiological signal may possess characteristics on which a determinationconcerning the possibility of a system fault may be based.

In some examples described herein, a first signal sensed by one or moreof electrodes 24, 26 of leads 20 is sensed within a first region of theCircuit of Papez. This first signal may be sensed sometime during orafter (e.g., immediately after or within some predetermined duration oftime thereafter) a relatively low frequency electrical stimulationsignal is delivered to a different second region of the Circuit ofPapez. Based on the sensed signal, a potential fault may be detected ina component of the medical system.

According to example techniques, a target site for deliveringstimulation within the second region of the brain in the mannerdiscussed above is the site selected for treating a seizure disorder ofpatient 12. This site may be selected by delivering a relatively lowfrequency stimulation (e.g., between 1 and 20 Hz, or between 2 and 10Hz) to a region of brain 28 and determining an evoked potential in aregion believed to be the seizure focus. For example, a relatively lowfrequency stimulation may be delivered to an area of the AN of brain 28and the local field potential (LFP) in the HC that is evoked by thedelivery of stimulation to the area of AN may be determined. This localfield potential may be sensed immediately, or after a short delay,following the delivery of stimulation to the area of AN, and may bereferred to as the evoked potential (EP). This evoked potential may becharacterized by the peak amplitude of a sensed bioelectrical brainsignal, which results when the brain activity in the thalamus propagatesto the HC. Thus, the evoked potential is an excitatory response of theHC to the stimulation delivered to the thalamus. The relatively lowfrequency stimulation within the AN generates a spike in brain activityin the HC. The target location for delivering stimulation may beselected as that location that results in an evoked potential in the HChaving a particular characteristic.

In example techniques, the stimulation site in the AN that results inthe greatest evoked potential in the HC (e.g., the brain signal with thegreatest average peak amplitude for a time period following the deliveryof the stimulation to the thalamus) is selected as the targetstimulation site. This is because the area of the AN associated with thegreatest evoked potential in the HC may have the strongest functionalconnection to the HC, such that delivery of stimulation to the area ofthe AN may provide efficacious stimulation therapy for managing theseizure disorder of patient 12. In other cases, the characteristic ofthe sensed response in the HC that is used to select the targetstimulation location in the AN may be some other characteristics. Thecharacteristic may be a frequency domain characteristic or a time domaincharacteristic of the bioelectrical brain signal. Examples of a timedomain characteristic include, but are not limited to, a mean, median,instantaneous, peak or lowest amplitude of the bioelectrical brainsignal within a predetermined period of time. Examples of a frequencydomain characteristic include, but are not limited to, a power level inone or more frequency bands of a bioelectrical brain signal sensed overa predetermined period of time or a ratio of power levels in at leasttwo frequency bands of the bioelectrical brain signal.

Once this target location for delivery of therapeutic stimulation hasbeen determined, a lead may be located at this site to provide chronicstimulation to treat the patient's disorder (e.g., seizure disorder).Another, second lead may be located within the AN to continue to monitorthe seizure focus location. According to aspects of this disclosure,these two leads may be used to detect the possible existence ofcomponent failures or faults within the medical device system. Forinstance, it may be known that a particular response is generated withinthe HC by stimulation delivered to the AN when no fault is present inthe system. That is, it may be known that a biological signal having aparticular waveform envelope, amplitude, duration, etc. may be expectedto be sensed in the HC when low-frequency stimulation is delivered tothe AN. This particular biological signal sensed within the HC may beunique to a given patient, or in some cases may be common to a givenpatient population that has a similar disorder, has a lead placementsimilar to that of the current patient, and so on. This biologicalsignal may be the signal that is expected to be sensed in the absence ofa system fault and may be considered a baseline signal for systemdiagnostic purposes. In one example, such a signal is recorded at a timewhen system components may be assumed, or are known, to be defect-free.For instance, this type of baseline signal may be recorded after it hasbeen determined that impedance measurements of system components are allwithin normal ranges and patient response to stimulation are as expectedfor an operational system. At such a time, the baseline biologicalsignal may be obtained and stored within a memory (e.g., a memory of IMD34) for later use.

In some cases, the baseline signal may be obtained by processingmultiple instances of a patient's measured response signal. Forinstance, at multiple times throughout a predetermined time period,stimulation (e.g., low frequency stimulation) may be delivered to the ANand an evoked response may be recorded in the HC. Signal processing maybe performed on the multiple signal recordings to generate a mean,median, or some other signal that is representative of, or a compositeof, the multiple signal recordings. In another example, the baselinesignal may be obtained, at least in part, from an expected responsederived from patient population data, wherein the population data isderived from patients having a similar demographic data (e.g., same sex,similar age, similar/same disorder and disease progression, etc.), asimilar system configuration, similar lead placement and/or arereceiving a similar therapy as compared to the current patient.

Sometime later after one or more baseline response signals have beenobtained, it may be determined that stimulation delivered to the AN isnot resulting in a signal that is the same as, or similar to, the samepreviously-obtained baseline biological signal. For instance,stimulation that is similar to, or the same as, that delivered to the ANto obtain the baseline signal may be delivered to the AN and a signalmay be sensed within the HC. At least a portion of that sensed signalmay be associated with delivery of stimulation to the AN. For instance,a portion of the sensed signal that is sensed at the same time as,and/or after some expected delay following stimulation of the AN may beassociated with the stimulation. This may be the portion of the sensedsignal in which the expected response may be located. Template matchingtechniques may be used to determine whether the stored baseline signalis similar to the newly-recorded signal, or whether one or morecharacteristics of the sensed signal are different from the storedbaseline signal. Characteristics used in the comparison may includeamplitude of the response, response duration, waveform morphology of theresponse, and so on. In other examples, frequency domain data may beused in addition to, or instead of, time domain data to compare thepreviously-recorded baseline signal to the newly-recorded response. Insuch cases, the power level in one or more frequency bands may becompared between the baseline and newly-recorded signal to determine ifdiscrepancies are evident. In some scenarios, a ratio between thebaseline and newly-recorded signal may be derived and compared to athreshold to determine if discrepancies are present.

If no fault exists in the system, it is likely that characteristics ofthe associated portion of the sensed signal will be similar to thebaseline signal such that a match is indicated.

However, if a fault exists in the system, the associated portion of thesensed signal may be degraded as compared to the baseline signal. Forinstance, the amplitude of the signal or the waveform characteristicsmay be degraded in comparison to those of the baseline biological signalas will be discussed further below. Alternatively, none of thecharacteristics of an evoked response may be present within theassociated portion of the signal, indicating that there is no detectableresponse present in this portion of the sensed signal at all. In such asituation, the expected response to the stimulation is not detected,pointing to the possibility that some component (a lead, lead extension,or an associated connector) in the path delivering stimulation ordetecting the response has a fault such as an open pathway preventingsignal conduction.

The foregoing describes a situation wherein a functional connectionbetween the AN and HC allows a signal sensed in the HC to detect anevoked response (or evoked potential) resulting from stimulationdelivered in the AN. The functional connection within the brain betweenthe AN and HC also allows stimulation delivered in the HC to result in aresponse that may be sensed within the AN. Thus, in a manner similar tothat described above for obtaining a baseline signal in the HC during atime when it is known that faults are absent in the system, a baselinesignal may additionally or alternatively be recorded or generated fromsignals sensed in the AN. For instance, a generated signal may bederived from one or more instances of signals sensed in the AN during,and/or shortly after, stimulation provided in the HC. Alternatively,some population data may be used to derive or obtain this baselinesignal. This baseline signal may be indicative of the expected responsewhen no fault is present in the system. This baseline signal may bestored within memory 62 of the IMD 16, memory of programmer 14, or someother system memory for later use.

Sometime later, the stored signal may be retrieved and compared to asignal recorded in the AN that is contemporaneous with stimulation inthe HC. If the later-obtain signal in the AN does not reflect thepresence of an evoked response at all, or signal characteristicsindicative of an evoked response have degraded, a potential fault mayhave occurred within the system. In the foregoing manner, functionalconnections within the brain such as those in the circuit of Papez mayallow stimulation at a first location and sensing at a second locationand/or stimulation at the second location and sensing at the firstlocation to be used to determine whether a fault may exist within thesystem.

Using sensing at both a first location and at a second location mayprovide more information than sensing at just one location. In otherwords, it may be advantageous to sense a possible evoked response at afirst location during and/or after stimulation at a second location aswell as to sense for a possible evoked response at the second locationduring and/or after stimulation at the first location. This additionalinformation may help to further analyze and pinpoint the location andnature of the potential fault as will be discussed further below.

While examples in which a target therapy delivery site within the AN ofbrain 28 is used in conjunction with sensing of a brain signal withinthe HC to detect a potential fault, in other examples, the techniquesdescribed herein may also be used to select a target stimulation sitewithin other regions of brain 28. For instance, in some cases,stimulation may be delivered to the subthalamic nucleus (STN) and asignal may be detected from the motor cortex using cortical electrodes.In such an example, electrocorticogram (ECoG) signals may be sensed bythe cortical electrodes to determine whether a response is detected as aresult of stimulation delivered to the STN.

Conversely, in some cases, the cortical electrodes may be used todeliver stimulation to the motor cortex to determine whether a signalsensed elsewhere within the brain, such as within the STN, is indicativeof an evoked response. In yet other examples, one or more electrodes maybe placed subgaleally between the skull and the scalp. For instance, aReveal LINQ® device commercially available from Medtronic, plc is abattery-powered loop recorder having a housing carrying multipleelectrodes. This device may be injected into the subgaleal space tosense a signal that may contain one or more characteristics of an evokedresponse occurring because of stimulation delivered at a differentlocation in the brain, such as stimulation delivered by lead 20 coupledto IMD 34. Such stimulation may be delivered to the STN or the AN of thethalamus, for example. The delivery of the stimulation by IMD may bewirelessly synchronized with recording by the LINQ device of the sub-Qsignal so that a portion of the recorded signal may be associated withthe stimulation. By such synchronization, the portion of the signal inwhich a potential response to the stimulation may be recorded andanalyzed (if such a response is indicated by the recorded signal). Therecorded signals may be stored within the LINQ device itself, and/or maybe wireless transferred to an external device such as programmer 14 oreven to IMD 34. Additionally or alternatively, such recordings may beuplinked to a “cloud-based” server system for analysis and retention.

In still other cases, one or more electrodes placed external to thepatient's body may be used to sense an EEG signal based on stimulationdelivered to a location in the brain. For instance, EEG electrodes thatare coupled (either in a wired or wireless manner) to a recording devicecan be used to record signals from the patient's scalp in a conventionalmanner. The recording of these signals can be synchronized tostimulation delivery in a manner similar to that described in theforegoing paragraph. Recordings may be compared to baseline EEGrecordings that were obtained when it was known that the system did notcontain faults. In a manner similar to any of the above-describedexamples, the EEG recordings may be used to determine whether anexpected evoked response signal is evidenced by signal characteristicspresent in a portion of the recorded signals that are associated withstimulation delivered to the brain. Such stimulation may be delivered byone or more electrodes 24, 26 of leads 20 that are coupled to IMD 34(FIG. 1). Stimulation may be delivered to the STN or AN of the thalamus,as one example, and recorded signals may be obtained from variouslocations on the scalp. The EEG electrodes may record what is known asan “EP recruitment rhythm” which is indicative of the response beingevoked by the stimulation.

In some examples, stimulation and sensing may occur within the samehemisphere of the brain, but this need not be the case. For example,stimulation in the HC of a first hemisphere of the brain can evoke aresponse in the HC of a second hemisphere via the dorsal hippocampalcommissure. In a manner similar to that discussed above, baselinesignals may be derived or recorded at a location of a second hemisphereat which recording will be performed for comparison to later-obtainedrecordings.

As may be appreciated from the foregoing, various combinations ofsensing and stimulation are possible, with stimulation occurring atlocations that are known to be functionally coupled to the sensinglocations. Such locations may be in the same, or different, hemispheresof the brain. Sensing and stimulation locations may comprise thosewithin deep brain structures, cortical structures, locations within thesubgaleal space, and locations on a surface of the body (e.g., thescalp).

In a manner described above, at least a portion of a signal sensed at afirst location may be associated with a signal introduced at a secondlocation, such as a stimulation signal delivered at a second location.The portion of the signal that is so associated may be analyzed todetermine whether characteristics of an expected evoked response signalare present in that signal portion, as may be expected if no faults haveoccurred within the system. If such characteristics are degraded, or notpresent at all, it may indicate a potential failure in the system.

As may be appreciated, when expected characteristics are either degradedor not present at all, a fault may exist within a stimulation path, suchas within a lead, lead extension and/or electrode(s) that deliver thestimulation and/or a fault may exist within a sensing path, such aswithin a lead, lead extension and/or electrode(s) that sense theresponse. To further determine in which path the fault may reside,additional information may be needed. For instance, in the case in whichcharacteristics of the expected evoked response signal are absent froman associated portion of the sensed signal, it may further be determinedwhether the sensed signal is indicated of physiological activity thatmay be expected to be obtained from the sensed location in the absenceof such a response. As a particular example, a signal sensed within theAN may reflect a relatively high level of theta activity. This thetaactivity will typically be present in the absence of stimulation in theHC and will result in a sensed signal having a characteristic signatureindication of such activity. If a signal sensed in the AN during, orjust after, stimulation is delivered to the HC is devoid of anycharacteristics of an expected evoked response due to the stimulationbut yet reflects the characteristic signature indication of thetaactivity expected in the AN, it may be determined that the fault lies inthe stimulation path. On the other hand, if both characteristics of theevoked response as well as characteristics of the expected theta wavesignature are missing from the signal sensed in the AN, it may be likelythat a fault lies in the sensing pathway.

Once a determination of the type described above has been made, thestimulation/sensing roles of the various lead/electrode pathways may bereversed so that, for instance, an electrode positioned within the ANdelivers the stimulation while an electrode in the HC senses acorresponding signal. If it had been determined in the first iterationdescribed in the foregoing paragraph that a fault likely exists in thepath involving the electrode(s) in the HC rather than in the AN, thensensing by that path in the second iteration will likely reveal degradedor non-existent characteristics of an evoked response, and will alsoindicate that the expected HC signal (that is, the signal that isexpected to be sensed in the absence of the evoked response) is alsodegraded, or does not include the expected signal characteristics.Conversely, if it had been determined in the first iteration that afault likely exists in the path involving the electrode(s) in the AN,then sensing by the electrode(s) in the HC will likely reveal degradedor non-existent characteristic of an evoked response, but will reflectthe expected HC signal characteristics. In this manner, switching thestimulation/sensing roles of the pathways helps to further confirm alikely location and cause (e.g., pathway) of a fault.

In some examples, a lead shift may be the result of a loss of anexpected evoked response signal rather than a system fault. In thiscase, an electrode combination that had previously been used to generatea baseline signature for use in detecting evoked responses may no longerbe as capable of sensing characteristics of such a response because thelead has shifted upward or downward in the anatomy. In such cases, it islikely that a different electrode combination may be able to sense thecharacteristics of the evoked response. This would likely not be thecase if the cause of the loss of the evoked potential signal wereinstead an open, short, or leakage path in the sensing pathway. Bycycling through different electrode combinations to obtain differentsense signals while stimulation is delivered at a second location in thebrain, it may be determined whether a system failure (e.g., failure in alead, lead extension, electrode, etc. in the sensing pathway) or a shiftin lead location is the cause of the signal loss.

In a similar manner, if it is determined that a likely fault may existsomewhere within a stimulation pathway, multiple different combinationsof available electrodes in the stimulation pathway may be tried todetermine if all such combinations fail to elicit a sensed evokedresponse in a sensing pathway. If all such combinations fail in thisregard, it may be likely that a fault exists in a component that iscommon to all electrode combinations, such as faulty connection betweena lead and lead extension 18, a lead and connector block 30, leadextension and connector block 30, and so on. If only some of thestimulation electrode combinations fail to result in an evoked response,the fault may likely be in some component that is not common to allcombinations such as a fault in one of the electrodes of the failingcombination or a failure in one or more conduction paths that are solelycoupled to the failing electrode combination(s).

As may be appreciated, various combinations of sensing and stimulation,including stimulation using various electrode combinations, may be usedto gather information that is useful to further analyze a likely faultoccurring in a component of a medical device system.

When a likely fault is detected, it may be desirable to further confirmand/or analyze the fault using additional signal data. For instance, itmay be desirable to evaluate a sensed signal, such as a biologicalsignal sensed in the brain, to determine whether some other signal thatoriginated elsewhere in the patient's body (e.g., an artifact) mayprovide more information on whether a potential fault has occurred in asystem component. As an example, a signal such as a local fieldpotential signal may be sensed within a location of a brain of a patientat a time the patient performs a motion task. This motion task mayinvolve the patient turning his or her head from side to side, ornodding his or her head up and down. It may instead involve swinging thearms around the torso, repeatedly bending at the waist or some othertask that may flex the lead extension or lead body. In another example,it may involve palpating the site of the IMD implant. This motion maycause a motion artifact that is present in the sensed signal (e.g.,local field potential signal) because of leakage pathways resulting frominsulation breaks or incomplete sealing of the system components. Suchartifacts may be intermittent and may coincide with a particularlocation of the head, for example, as the patient continues with headrotation. Such artifacts may evidence a regular frequency that coincideswith the frequency at which the patient is moving, further providingconfirmation of a break/open or a short that is being exposed by themotion. An artifact of this type may be a notable spike, or amplitudeincrease, that occurs at a regular frequency and/or time associated withthe motion. This likely time correlation with the movement will providean indication that this activity is not associated with a physiologicalsignal (e.g., not the result of a seizure or after discharge event) butrather is associated with a component fault in the system.

Such motion artifacts in a sensed signal may correspond to a particularportion of the sensed signal that is associated with the motion. Forinstance, a local field potential signal may be sensed over apredetermined time period. The patient may be directed to enter intomotion only during a portion of this time period. The portion of thesensed signal obtained during the time when motion was occurring may becompared to the portion of the sensed signal when no motion wasoccurring to determine if motion artifacts are present in the signal. Insome cases, other characteristics of the signal may be analyzed withrespect to the motion. As discussed above, the analysis may determinewhether a frequency at which the patient is turning his or her headcorresponds to a frequency of a characteristic of the sensed signal. Inthis manner, it may be determined whether a motion artifact has beenintroduced into the sensed signal such that a component failure mayexist.

In the foregoing manner, one or more processors within the system areconfigured to associated a portion of the sensed signal (e.g., the LFPsignal sensed in the brain during motion of the patient) with a secondsignal introduced at a second location of the anatomy (e.g., a motionsignal resulting from twisting of the torso, turning of the head, etc.).Characteristics of the sensed signal may be indicative of whether afault exists in the system. As discussed above, such characteristics maycomprise characteristics obtained from a sensed time-domain signal, suchas an increased amplitude of a portion the signal or an envelope of thesignal. This may comprise, for instance, characteristic signal spikes.Alternatively or additionally, a duration of the signal may beindicative of the fault. For instance, a very narrow spike may occurbecause of an intermittent fault that manifests itself only during ashort portion of the motion. As another example, a frequency of thesignal (e.g., the frequency at which spikes occur) may coincide withfrequency of motion and may provide another confirmation that the signalindeed is the result of the motion.

In some cases, a baseline time domain signal may be obtained at a timewhen the system is known to be free of faults. As in the case discussedabove related to evoked responses, this baseline time domain signal maybe obtained while the patient is moving but while no faults aresuspected to exist within the system. Such a signal, which may be an LFPsignal sensed from an electrode within a patient's brain, may be stored,for instance, in a memory within the system for comparison to alater-obtained signal that is obtained when some fault may be presentwithin the system. Comparison of the later-obtained signal to thisbaseline may be used to determine whether a fault exists. Additional oralternatively, characteristics of the sensed signal obtained duringpatient motion may comprise those determined based on a frequency domainsignal. That is, a transformation such as a FFT transformation may beperformed on the sensed time domain signal (e.g., the LFP signal) togenerate a frequency domain signal. A characteristic of that frequencydomain signal may be used to determine whether a fault exists. Forinstance, a notable increase in the power level of a particularfrequency or frequency band that is present during motion but absentwhen the patient is not moving may indicate the presence of an open orshort in the system component. In this manner, characteristics from thetime- and/or frequency-domain signals may be used to perform faultanalysis.

In some examples, a baseline frequency domain signal may be obtainedfrom the baseline time-domain signal. Such a signal may be indicative ofthe signal obtained during patient motion but when it is known that nofault is present in the system. Such a signal may be obtained, forinstance, by performing a transformation such as an FFT transformationon a time-domain signal LFP sensed while a patient was undergoing motionand while it is known that the system is free of faults. This baselinefrequency domain signal may be compared to the frequency domain signalobtained later when a fault may be present in the system. Comparisonbetween the baseline and the later-obtained signal may be used toanalyze whether the system may be subject to a fault.

Although the foregoing discussed use of LFP signals obtained duringpatient movement to analyze a failure in one or more components (orinterconnections between components) of a system, other types of signalsmay be used for this purpose, such as EEG signals, ECoG signals, and/orany of the other types of signals described herein or otherwise known tobe suitable in regards to evoked potentials.

The foregoing discusses use of patient movement to introduce a signalinto the patient's body that may be used to further analyze systemfaults. Still other types of signals may be introduced into the body forthis purpose. By way of another example, and in a manner similar to theforegoing, a brain signal such as an LFP signal may be sensed todetermine whether it exhibits characteristics of a cardiac activity,such as a cardiac artifact (e.g., an ECG artifact.) Generally, when nofault has occurred within the system, electrodes 24, 26 will not exhibitcharacteristics of cardiac activity. However, when fluid ingress hascaused a leakage pathway into the system, the relatively large waveforms(e.g., ECG waveforms) generated by electrical activity of the heartcontaminate or completely mask out the relatively small neural signalsthat would otherwise be sensed by electrodes 24, 26 in the brain. Thisis because the cardiac signals have amplitudes in the millivolt range,whereas neural signals have a much smaller amplitude, generally in themicrovolt range). The presence of an ECG signal that is overlaying, orcompletely masking a neural signal may be an indication of a leakagepathway in a header block of IMD 34, in a connector that couples a lead20 to a lead extension 18, a pathway resulting from a breach in a leador lead extension insulating body, and so on.

Other larger signals introduced into the patient's body may likewise byused in addition to motion artifacts and cardiac activity artifacts todetermine the existence of faults in the system. For instance, “tapping”on the housing of the IMD located under the skin may generate arelatively large signal that will travel through the body and be pickedup by a system component if a fault is present. Such a signal willlikely not be sensed within a brain signal if no such fault exists.Similar types of introduced signals may also be used for this purpose,such as asking the patient to clap their hands, and so on. Thus, avariety of different types of signals may be introduced at a secondlocation within the body while sensing is occurring a different firstlocation in the body (e.g., within or on the brain) that can be used toperform fault analysis.

As may be appreciated, when any of the types of signals are introducedinto the body, it may be beneficial to determine whether only one, orboth leads in a dual lead system, are detecting any resulting artifacts.If both leads are detecting these artifacts, it may be an indicationthat a header block of IMD 34 has experienced a leakage path that isaffecting both sensing pathways provided by both leads. On the otherhand, if only one of the leads is detecting these artifacts, the faultis likely in a component of that pathway (e.g., the lead, leadextension, a connector component affecting connections in this pathway,or some other component in the pathway.)

The aforementioned techniques provide mechanisms for detecting a faultin an implantable medical device system. According to an example method,a first signal is sensed via a sensor (e.g., an electrode) at a firstlocation of an anatomy of a patient. A portion of this first signal isassociated with a second signal introduced at a second location of theanatomy of the patient. This second signal introduced at a secondlocation may comprise a stimulation signal, a motion artifact, a cardiacartifact, or any other signal that may be present or introduced in thepatient's body that may result in signal characteristics sensed in thesignal sensed at the first location. In one example, a portion of thefirst signal is associated with the second signal by time-correlatingthe occurrence of the second signal with the portion of the firstsignal. That is, a portion of the first signal is so associated if thatportion is obtained contemporaneously with the occurrence of the secondsignal (either at the same time or at a time thereafter, where this timemay be associated with an expected delay in the transmission of thesignal from the second to first location.) The method further comprisesdetermining whether a fault exists in the system based on one or morecharacteristics of the associated portion of the first signal. Forinstance, such characteristics may indicate a loss or degradation of anexpected evoked response signal that would otherwise have been presentin the sensed signal. Such a loss or degradation can be detected, forinstance, by comparison of the associated portion of the first signalwith a baseline signal, which may be a signal stored in memory of IMD34, programmer 14, or stored somewhere else and used as a template forcomparison against the first signal. The characteristics may be time- orfrequency-domain characteristics.

In another example, a system is disclosed that comprises a sensorconfigured to sense a first signal at a first location of an anatomy ofa patient, and one or more processors configured to associate a portionof the first signal with a second signal introduced at a second locationof the anatomy of the patient and to determine whether a fault exists inthe system based on one or more characteristics of the associatedportion of the first signal.

In some examples, it may be beneficial to utilize impedance measurementsin conjunction with sensed signals for use in system faultdeterminations. Impedance measurements may be used to determine theimpedance existing within a particular sensing pathway, for instance.Such a measurement takes into account the tissue impedance as well asimpedance resulting from the components in that pathway, such as theconductors, the electrodes, and any other circuit components in thatpathway. One example way in which to determine such an impedance is forIMD 34 to output a current via electrodes 24, 26 coupled to the tissue,and to then determine a voltage caused by the flow of current throughthe tissue. The voltage value may be divided by the current to determinethe tissue resistance. Such operations may be performed, for example, byprocessor 60 of IMD 34 (FIG. 3). Conversely, the IMD 34 may output avoltage via electrodes 24, 26 coupled to the tissue, determine a currentcaused by the voltage applied to the tissue, and divide the voltage bythe current to determine the impedance of the path.

Various techniques, circuits, and systems for sensing and measuringimpedance generally, and in particular for measuring impedance inmedical applications, are described in U.S. Patent Application No.62/084,252 entitled “Tissue Resistance Measurement” filed Nov. 25, 2014and U.S. patent application Ser. No. 14/952,675 entitled “TissueResistance Measurement” filed Nov. 25, 2015. Such techniques, circuits,and system are further described in U.S. patent application Ser. No.12/872,552 filed Aug. 31, 2010, now U.S. Pat. No. 9,615,744, and U.S.Pat. Nos. 7,391,257, 7,622,988, and 9,197,173, all of which are entitled“Chopper-Stabilized Instrumentation Amplifier for Impedance Measurement”and all of which are assigned to the assignee of the present disclosure.All of the foregoing documents are incorporated herein by reference intheir entireties.

In some of the example impedance sensing circuits referenced herein, theimpedance measurements can be taken in substantially real-time. That is,a succession or sequence of impedance values may be obtained over timeas, for instance, the patient is undergoing motion. The impedancemeasurements may be time-correlated with the patient's motion so thatopens and shorts occurring as the patient moves will be reflected in thetime-sequence of measurements obtained for a particular circuit pathway(e.g., the pathway including multiple electrodes and the conductorscoupled to these electrodes.)

As a particular example of the foregoing, a user such as a clinician orpatient may provide input such as by interacting with a user interfaceof a programmer or some other external device and/or by tapping on theIMD 34 in a way that may be detected by an on-board accelerometer. Suchuser input may be provided at a time that corresponds with the turningof a patient's head or some other movement. Such input may cause atimestamp to be stored in memory to indicate time(s) of patient motion.Corresponding timestamps may be introduced into a stream of impedancevalues by a system clock (e.g., a clock of IMD 34 and/or a clock ofprogrammer 14). If such impedance values are obtained in real-time orsubstantially in real-time, the timestamps introduced into the stream ofmeasurement data may be correlated to the stored timestamps associatedwith patient movement. In this way, it may be possible to determine thateach time a patient performs a certain type of motion an out-of-rangeimpedance value is detected on one or more sets of electrodes 24, 26.Such an out-of-range value may indicate an unusually high impedancevalue (e.g. an impedance value that is above some selected impedancevalue threshold for the particular system) or an unusually low impedancevalue (e.g., an impedance value that is below some selected impedancevalue threshold for the system). In some cases, these high and lowthreshold values may be programmable or otherwise selectable values, andin other cases these may be determined by, for instance, a supplier ofthe medical system or a clinician. In any event, a stream or sequence ofsuch impedance values can be used to confirm the suspected presence ofan open or short in the system, particularly when coupled with otherinformation such as that described above.

A time-correlated stream, or sequence, of impedance values obtained inreal- or substantially real-time may be stored in memory of IMD 34,stored within programmer 14, stored in some other external device (e.g.,a clinician workstation a cell phone or PDA, or some other device),and/or uploaded to “the cloud” for storage on a central database. Thisdata may further be used to generate information that may be presentedto a clinician. For instance, a display on a screen of programmer 14 maybe used to illustrate in a graphical or other format the variation inimpedance over time. This graphical or other display could be annotatedto indicate the times at which patient movement occurred (e.g., thetimes of head movement) so that the clinician can determine whether alikely short or open is occurring intermittently with some type ofmovement.

A display of the type discussed above may also be correlated (e.g., on acommon time axis) with a graphical representation of the patient'smovement (e.g., a representation of a head performing rotationalmovements at the same frequency as was performed by the patient) alongwith a rolling window displaying the stream of impedance data so that aclinician can determine a likely rotational position of a patient's headat the time of an intermittent short of open, and thereby aid in thediagnosis of the system fault.

In some examples, graphical motion and/or impedance data may bepresented on a common timeline with sensed data of the type discussedabove. Sensed data may include LFPs, ECoGs, EEGs or some otherphysiological sensed signal that may reflect a motion artifact resultingfrom the patient's movement. If such data has been time-stamped duringcollection so that it can be cross-correlated with the stream ofimpedance data, a clinician may be able to readily correlate high- orlow-impedance values that occur at the same time as (or substantiallythe same time as) as motion artifacts that are present in the sensedsignals. Again, such motion artifacts may be manifested as short,high-amplitude, spikes in the system that are non-physiological innature. Such signals may be characterized as non-physiological becauseof their consistent non-physiological frequency (e.g., corresponding tofrequency of patient motion rather than any frequency naturallyoccurring within the physiological signal). This may allow the user tomore readily determine the likelihood of a fault, and may providefurther confirmation of a motion-induced open or short in the system.

Other types of information may also be useful in further diagnosing apotential system fault. For instance, it may be determined whether apatient has experienced a potential loss in therapy efficacy. As oneexample, a patient suffering from Parkinson's disease may be receivingstimulation to an area of the STN to treat symptoms such as bradykinesiaor dyskinesia. The clinician may note that based on recorded therapymodification made by the patient to stimulation amplitude, as may berecorded in an electronic therapy diary stored within memory of IMD 34,the patient has systematically requested ever-increasing stimulationamplitudes while obtain less relief from their symptoms. This loss ordegradation in therapy efficacy may point to a short or open in apathway providing the stimulation to the STN. When taken in conjunctionwith other information gathered according to one or more of theaforementioned techniques, this information involving a change intherapy efficacy may be used to pinpoint the source of the failure.Returning now to a discussion of FIG. 1, IMD 16 may deliver therapy tothe brain 28 in a manner that influences the brain signals within one ormore regions of brain 28. For example, IMD 16 may deliver therapy to theAN, HC, STN, or other suitable region of brain 28 to control a brainstate of patient 12 in a manner that effectively treats a disorder ofpatient 12. For example, in the case of a seizure disorder, IMD 16 maydeliver therapy to a region of brain 28 via a selected subset ofelectrodes 24, 26 (referred to herein as an electrode combination) tosuppress a level of brain activity within the AN, HC, or another brainregion associated with the occurrence of seizures (e.g., a seizure focusof brain 28). IMD 16 may deliver therapy to brain 28 via a selectedsubset of electrodes 24, 26.

Although leads 20 are shown in FIG. 1 as being coupled to a common leadextension 18, in other examples, leads 20 may be coupled to IMD 16 viaseparate lead extensions or directly coupled to IMD 16. Moreover,although FIG. 1 illustrates system 10 as including two leads 20A and 20Bcoupled to IMD 16 via lead extension 18, in some examples, system 10 mayinclude one lead or more than two leads.

In the examples shown in FIG. 1, electrodes 24, 26 of leads 20 are shownas ring electrodes. Ring electrodes may be relatively easy to programand are typically capable of delivering an electrical field to anytissue adjacent to leads 20. In other examples, electrodes 24, 26 ofleads 20 may have different configurations. For example, electrodes 24,26 of leads 20 may have a complex electrode array geometry that iscapable of producing shaped electrical fields. The complex electrodearray geometry may include multiple electrodes (e.g., partial ring orsegmented electrodes) alone, or in combination with ring electrodes. Thepartial ring or segmented electrodes may be provided around theperimeter of each lead 20, (in contrast to a full ring electrode whichextends around the entire circumference of the lead body.) In thismanner, electrical stimulation may be directed to a specific directionfrom leads 20 to enhance therapy efficacy and reduce possible adverseside effects from stimulating a large volume of tissue. In someexamples, outer housing 34 of IMD 16 may include one or more stimulationand/or sensing electrodes. For example, housing 34 can comprise anelectrically conductive material that is exposed to tissue of patient 12when IMD 16 is implanted in patient 12, or an electrode can be attachedto housing 34. In other examples, one or both leads 20 may have a shapeother than elongated cylinders as shown in FIG. 1. For example, leads 20may be paddle leads, spherical leads, bendable leads, or any other typeof shape effective in treating patient 12.

External programmer 14 is configured to wirelessly communicate with IMD16 as needed to provide or retrieve therapy information. Programmer 14is an external computing device that the user, e.g., the clinicianand/or patient 12, may use to communicate with IMD 16. For example,programmer 14 may be a clinician programmer that the clinician uses tocommunicate with IMD 16 and program one or more therapy programs for IMD16. Alternatively, programmer 14 may be a patient programmer that allowspatient 12 to select programs and/or view and modify therapy parameters.The clinician programmer may include more programming features than thepatient programmer. In other words, more complex or sensitive tasks mayonly be allowed by the clinician programmer to prevent an untrainedpatient from making undesired changes to IMD 16. Programmer 14 may be ahand-held computing device with a display viewable by the user and aninterface for providing input to programmer 14 (i.e., a user inputmechanism).

For example, programmer 14 may include a small display screen (e.g., aliquid crystal display (LCD) or a light emitting diode (LED) display)that presents information to the user. In addition, programmer 14 mayinclude a touch screen display, keypad, buttons, a peripheral pointingdevice or another input mechanism that allows the user to provide input.If programmer 14 includes buttons and a keypad, the buttons may bededicated to performing a certain function, e.g., a power button, or thebuttons and the keypad may be soft keys that change in functiondepending upon the section of the user interface currently viewed by theuser. In some examples, the screen (not shown) of programmer 14 may be atouch screen that allows the user to provide input directly to the userinterface shown on the display. The user may use a stylus or theirfinger to provide input to the display.

In other examples, programmer 14 may be a larger workstation or aseparate application within another multi-function device, rather than adedicated computing device. For example, the multi-function device maybe a notebook computer, tablet computer, workstation, cellular phone,personal digital assistant or another computing device that may run anapplication that enables the computing device to operate as a securemedical device programmer 14. A wireless adapter coupled to thecomputing device may enable secure communication between the computingdevice and IMD 16.

When programmer 14 is configured for use by the clinician, programmer 14may be used to transmit initial programming information to IMD 16. Thisinitial information may include hardware information, such as the typeof leads 20, the arrangement of electrodes 24, 26 on leads 20, theposition of leads 20 within brain 28, initial programs defining therapyparameter values, and any other information that may be useful forprogramming into IMD 16. Programmer 14 may also be capable of completingfunctional tests (e.g., measuring the impedance of electrodes 24, 26 ofleads 20).

The clinician may also store therapy programs within IMD 16 with the aidof programmer 14. During a programming session, the clinician maydetermine one or more therapy programs that may provide efficacioustherapy to patient 12 to address symptoms associated with the seizuredisorder (or other patient condition). During the programming session,patient 12 may provide feedback to the clinician as to the efficacy ofthe specific program being evaluated or the clinician may evaluate theefficacy based on one or more physiological parameters of patient (e.g.,heart rate, respiratory rate or muscle activity).

Programmer 14 may also assist the clinician in the analysis of faultinformation of the type described herein. For example, sensed signalssensed by lead 20 and electrodes 24, 26 may be transmitted via telemetrytransmissions to programmer 14 for storage and analysis. Programmer 14may display portions of the sensed waveforms for viewing by a user. Insome examples, multiple types of data may be displayed at once. Forinstance, in systems in which a stream of impedance values are obtainedin real- or substantially real-time while a patient is moving, thisstream of impedance values may be displayed along a common timeline thatis annotated with times of patient motion and that further correlates asensed LFP or some other sensed physiological signal with the impedancedata. Such a display may show how periodic “blips” that appear in thewaveform are time-correlated with out-of-range impedance values, therebyconfirming that a failure within a system component is resulting insignal characteristics within the sensed physiological signal.

Other types of interfaces may be provided, such as an interface todisplay a waveform representative of delivered stimulation at a secondlocation that is time-correlated with an associated portion of a sensedsignal that is sensed at a time during or somewhat after, thestimulation. The sensed signal may be overlaid in one example with abaseline signal to allow for easy comparison between the two. This mayallow a clinician to determine the likelihood of a fault within thesystem.

Programmer 14 may also be configured for use by patient 12 in someexamples. When configured as a patient programmer, programmer 14 mayhave limited functionality (compared to a clinician programmer) in orderto prevent patient 12 from altering critical functions of IMD 16 orapplications that may be detrimental to patient 12. In this manner,programmer 14 may only allow patient 12 to adjust values for certaintherapy parameters or set an available range of values for a particulartherapy parameter.

Whether programmer 14 is configured for clinician or patient use,programmer 14 is configured to communicate to IMD 16 and, optionally,another computing device, via wireless communication. Programmer 14, forexample, may be configured to communicate via wireless communicationwith IMD 16 using radio frequency (RF) telemetry techniques known in theart. Programmer 14 may also communicate with another programmer orcomputing device via a wired or wireless connection using any of avariety of local wireless communication techniques, such as RFcommunication according to the 802.11 or Bluetooth specification sets,infrared (IR) communication according to the IRDA specification set, orother standard or proprietary telemetry protocols. Programmer 14 mayalso communicate with other programming or computing devices viaexchange of removable media, such as magnetic or optical disks, memorycards or memory sticks. Further, programmer 14 may communicate with IMD16 and another programmer via remote telemetry techniques known in theart, communicating via a local area network (LAN), wide area network(WAN), public switched telephone network (PSTN), or cellular telephonenetwork, for example.

Therapy system 10 may be implemented to provide chronic stimulationtherapy to patient 12 over the course of several months or years.However, system 10 may also be employed on a trial basis to evaluatetherapy before committing to full implantation. If implementedtemporarily, some components of system 10 may not be implanted withinpatient 12. For example, patient 12 may be fitted with an externalmedical device, such as a trial stimulator, rather than IMD 16. Theexternal medical device may be coupled to percutaneous leads or toimplanted leads via a percutaneous extension. If the trial stimulatorindicates DBS system 10 provides effective treatment to patient 12, theclinician may implant a chronic stimulator within patient 12 forrelatively long-term treatment. In addition, the trial stimulator can beused to select a target therapy delivery site for patient 12.

FIG. 2 is a conceptual diagram illustrating an example therapy system 50for delivery of a therapeutic agent to a tissue site within brain 28 ofa patient 12. Therapy system 50 includes IMD 52 and catheter 54, whichincludes a plurality of electrodes 56 for sensing one or morebioelectrical brain signals within brain 28 of patient 12. IMD 52 isconfigured to deliver at least one therapeutic agent, such as apharmaceutical agent (e.g., anti-seizure medication), anti-inflammatoryagent, gene therapy agent, or the like, to a target tissue site withinbrain 28 of patient 12 via catheter 54, which is in fluid communicationwith IMD 52. Catheter 54 may be coupled to IMD 52 either directly orwith the aid of an extension (not shown in FIG. 1).

In some examples, IMD 52 includes a fluid pump or another device thatdelivers a therapeutic agent in some metered or other desired flowdosage to the therapy site within patient 12 from a reservoir within IMD52 via catheter 54. For treatment of a seizure disorder, drug therapymay be intended to minimize the severity, duration or frequency ofseizures. Examples of pharmaceutical agents that IMD 52 may deliver topatient 12 to manage a seizure include, but are not limited to,adenosine, lorazepam, carbamazepine, oxcarbazepine, valproate,divalproex sodium, acetazolamide, diazepam, phenytoin, phenytoin sodium,felbamate, tiagabine, levetiracetam, clonazepam, lamotrigine, primidone,gabapentin, phenobarbital, topiramate, clorazepate, ethosuximide, andzonisamide. In other examples, IMD 52 delivers a therapeutic agent totissue sites within patient 12 other than brain 28. Electrodes 56 areconfigured to sense bioelectrical signals within brain 28 of patient 12to allow system 50 to monitor one or more bioelectrical brain signalswithin brain 28. In some examples, electrodes 56 may be substantiallysimilar to one or more of electrodes 24, 26 (FIG. 1). Although FIG. 2illustrates catheter 54 including four sense electrodes 56, in otherexamples, a catheter may include any suitable number of senseelectrodes, such as one, two, three or greater than four. In addition,although sense electrodes 56 are located proximal to the fluid deliveryport 55 of catheter 54 in the example shown in FIG. 2, in otherexamples, one or more of sense electrodes 56 may be distal to fluiddelivery port 55 of catheter 54. Catheter 54 may include more than onefluid delivery port. Thus, in some examples, one or more senseelectrodes 56 may be located between fluid delivery ports of catheter54.

In some embodiments, another therapy delivery device, such as a lead oranother catheter carrying electrodes, may be located at a differentregion of the patient's body that is remote from fluid delivery port 55.This second therapy delivery device may sense a response to delivery ofa therapeutic agent by catheter 54 and/or delivery of electricalstimulation by electrodes 56. The sensing of the response may be similarto sensing of the evoked response caused by delivery of electricalstimulation as discussed above. Characteristics of this sensed responsemay be used to determine whether a fault may exist in the system usingtechniques similar to those discussed above in regards to FIG. 1. Thus,therapeutic stimulation may comprise electrical stimulation, stimulationby a therapeutic agent, or some other type of stimulation, such asoptical or ultrasound stimulation. FIG. 3 is functional block diagramillustrating components of IMD 16. In the example shown in FIG. 3, IMD16 includes processor 60, memory 62, stimulation generator 64, sensingmodule 66, switch module 68, telemetry module 70, and power source 72.Memory 62 may include any volatile or non-volatile media, such as arandom access memory (RAM), read only memory (ROM), non-volatile RAM(NVRAM), electrically erasable programmable ROM (EEPROM), flash memory,and the like. Memory 62 may store computer-readable instructions that,when executed by processor 60, cause IMD 16 to perform various functionsdescribed herein.

In the example shown in FIG. 3, memory 62 stores therapy programs 74 andoperating instructions 78, which may be stored in separate memorieswithin memory 62 or separate areas within memory 62. Each stored therapyprogram 74 defines a particular program of therapy in terms ofrespective values for electrical stimulation parameters, such as astimulation electrode combination, electrode polarity, current orvoltage amplitude, and, if stimulation generator 64 generates anddelivers stimulation pulses, the therapy programs may define values fora pulse width, and pulse rate of a stimulation signal. In examples whenIMD 16 delivers electrical stimulation therapy on a cyclic basis (ascompared to a substantially continuous basis), memory 62 stores, e.g.,as part of therapy programs 74, cycle parameter information, such as, oncycle time duration and off cycle duration. In some examples, thetherapy programs may be stored as a therapy group, which defines a setof therapy programs with which stimulation may be generated. Thestimulation signals defined by the therapy programs of the therapy groupmay be delivered together on an overlapping or non-overlapping (e.g.,time-interleaved) basis.

Operating instructions 78 guide general operation of IMD 16 undercontrol of processor 60 and may comprise instructions to control howsignals are sensed from electrodes 24, 26, and how the signals arethereafter processed to determine fault detection, and so on.

In some example, processor 60 and/or a processor of programmer 14 mayperform the steps to perform the methods disclosed herein, such thatthese methods may be partially, or fully, automated. Thus, while in somecases, analysis of fault data may be performed in whole or in part, by aclinician, in other cases, such analysis may be performed according totechniques described herein in whole or in part may one or moreprocessors including processor 60 of IMD 16.

Sensing information 76 is also stored by memory 62. This information mayinclude evoked potentials (EPs) 76A resulting from sensing the result ofdelivered stimulation, and may include information pertaining to whetheror not characteristics of expected evoked potentials (or evokedresponses) are present within the sensed signals. The evoked potentialscould be LFP signals, or any other type of signals that indicate theresponse to stimulation. Sensing information 76 may further includemotion data 76B, which is data that may result from sensing duringperiods of patient motion and may include information pertaining towhether a motion artifact is reflected in the sensed signal. Suchinformation may further include data pertaining to when and what typesof motion were associated with the sensed data. Sensing information 76may further include ECG data 76C that may be sensed to determine whetheran ECG artifact is present in a sensed signal, such as a sensed brainsignal. These are examples of data only, and other artifact data may bestored instead of, or in addition to, the illustrated data, includingartifact data related to tapping on the housing of the IMD 16.

Memory 62 may also store baselines 77, which may provide baseline evokedresponse signals, and further may provide baseline signals forstructures in which electrodes 24, 26 are located, such as baselinesignals for the AN, HC, and STN. Such signals may be indicative of thosesignals expected to be received in the corresponding structures in theabsence of an evoked potential signal. These signals may be compared,for instance, to sensed signals to determine whether, in the absence ofan evoked potential signal, an expected signal is being received on aparticular electrode combination. This may be useful in furtherperforming fault isolation using techniques described herein. When usedtogether, these two types of baseline signals may be used to determinewhether a fault is likely in a stimulation or a sensing path asdiscussed above.

Baselines 77 may also include other baseline signals, such as baselinesignals expected to be obtained during a particular patient motion inthe absence of a system fault. In some examples, these types of baselinesignals are the same as the baseline signals obtained for the variousbrain structures (AN, HC, STN, etc.) in the absence of an evokedpotential. That is, if the motion does not have any effect on the sensedbrain signal in the absence of the fault, there is no need for thisadditional baseline signal, and the baseline signals discussed above maybe used as a comparison to those obtained during motion.

Memory may also store templates and thresholds 76D that may be used tocompare to evoked potential signals. Templates might provide thewaveform shape or a signature expected to be associated with aparticular type of fault. If template matching techniques indicate thistemplate does favorably compare, or “match” with a sensed signal that isobtained during motion, a fault may exist within the system.

Template and threshold data 76D may further include thresholds. In oneexample, a threshold may indicate an upper or lower bound of theamplitude of a signal (either in the time- or frequency-domain) thatwould be expected to be sensed in the absence of a system fault, such asa voltage, current, or power-level amplitude. In some cases, thethreshold may indicate an upper or lower bound of a value determined asa ratio between an amplitude of the sensed signal to that of thebaseline signal. In some cases, the threshold may indicate a power-levelof a frequency domain signal, or a duration (e.g., width of a signalcharacteristic) in a time-domain signal. Other types of thresholds arepossible within the scope of this disclosure. Also shown in FIG. 3 isstimulation generator 64. Under the control of processor 60, stimulationgenerator 64 generates stimulation signals for delivery to patient 12via selected combinations of electrodes 24, 26. In some examples, duringtherapy delivery to manage a disorder, stimulation generator 64generates and delivers stimulation signals to one or more target regionsof brain 28 (FIG. 1), e.g., the AN, of patient 12 via a selectcombination of electrodes 24, 26 (referred to herein as a stimulationelectrode combination) where the stimulation signals have a frequency ina range of about 1 Hertz (Hz) to about 10,000 Hz (or in some cases,between 2 Hz and 1200 Hz), a voltage of about 0.1 volts to about 10.5volts, and a pulse width of about 60 microseconds to about 450microseconds. In some examples, the stimulation signals have a frequencyof 120 Hz, a voltage of about 4 volts, and a pulse width of about 100microseconds. In addition, in some examples, the stimulation signalshave a frequency of 145 Hz, a voltage of about 5 volts, and a pulsewidth of about 145 microseconds.

In some examples, stimulation is delivered to generate an evokedpotential that can be associated with a portion of a sensed signal thatis sensed at a location other than that at which the stimulation wasdelivered. The associated portion of the sensed signal may then be usedto diagnose a system fault. The stimulation signals used for thispurpose may be low frequency stimulation signals, such a stimulationhaving a frequency of between 2 and 10 Hz.

As a specific example, stimulus frequencies below approximately 40 Hz(such as, e.g., between approximately 1 Hz and 40 Hz, or betweenapproximately 2 Hz and 10 Hz, may be delivered to the AN to evoke (or totry to evoke) a potential in the HC or vice versa. Any evoked potentialmay be sensed to determine whether the sensed signal, or a portion ofthe sensed signal that has been associated with the stimulation,exhibits an expected morpholophy, amplitude, and so on. This may be doneby comparing the associated portion of the sensed signal to a baselinesignal representing the shape, amplitude, duration, and so on of asignal that would be sensed if no fault existed within the system, asdiscussed above. In other examples, the associated portion of the sensedsignal may be analyzed to determine whether it meets one or morepredetermined criteria, such as whether the portion of the signal has acharacteristic having an amplitude, width, frequency, a power-level inone or more frequency bands, or some other predetermined characteristicof at least a predetermined threshold, a characteristic with atime-duration that meets some length criteria, a power level in one ormore frequency bands that is within some predetermined amplitude rangeor is above a predetermined power level, a waveform shape thatsubstantially matches some expected shape, and so on. The predeterminedcriteria may be determined using template-matching techniques.

Processor 60 may include any one or more of a microprocessor, acontroller, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field-programmable gate array (FPGA),discrete logic circuitry, and the functions attributed to processor 60herein may be embodied as firmware, hardware, software or anycombination thereof. Processor 60 is configured to control stimulationgenerator 64 according to therapy programs 74 stored in memory 62 toapply particular stimulation parameter values specified by one or moreprograms, such as amplitude, pulse width, and pulse rate.

In the example shown in FIG. 3, the set of electrodes 24 of lead 20Aincludes electrodes 24A, 24B, 24C, and 24D, and the set of electrodes 26of lead 20B includes electrodes 26A, 26B, 26C, and 26D. Processor 60 maycontrol switch module 68 to apply the stimulation signals generated bystimulation generator 64 to selected combinations of electrodes 24, 26.In particular, switch module 68 may couple stimulation signals toselected conductors within leads 20, which, in turn, deliver thestimulation signals across the electrodes 24, 26 of the selectedstimulation electrode combination. Switch module 68 may be a switcharray, switch matrix, multiplexer, or any other type of switching moduleconfigured to selectively couple stimulation energy to selectedelectrodes 24, 26 and to selectively sense bioelectrical brain signalswith selected electrodes 24, 26. Hence, stimulation generator 64 iscoupled to electrodes 24, 26 via switch module 68 and conductors withinleads 20. In some examples, however, IMD 16 does not include switchmodule 68.

Stimulation generator 64 may be a single channel or multi-channelstimulation generator. In particular, stimulation generator 64 may becapable of delivering, a single stimulation pulse, multiple stimulationpulses or continuous signal at a given time via a single electrodecombination or multiple stimulation pulses at a given time via multipleelectrode combinations. In some examples, however, stimulation generator64 and switch module 68 may be configured to deliver multiple channelson a time-interleaved basis. For example, switch module 68 may serve totime divide the output of stimulation generator 64 across differentelectrode combinations at different times to deliver multiple programsor channels of stimulation energy to patient 12.

Sensing module 66 is configured to sense bioelectrical brain signals ofpatient 12 via a sense electrode combination, which can include aselected subset of electrodes 24, 26 or with one or more electrodes 24,26 and at least a portion of a conductive outer housing 34 of IMD 16, anelectrode on an outer housing of IMD 16 or another reference. Processor60 may control switch module 68 to electrically connect sensing module66 to selected electrodes 24, 26. In this way, sensing module 66 mayselectively sense bioelectrical brain signals with differentcombinations of electrodes 24, 26 (and/or a reference other than anelectrode 24, 26).

In some examples, processor 60 may select an electrode combination fordelivering efficacious stimulation therapy to patient 12 based on one ormore characteristics of the bioelectrical brain signals monitored bysensing module 66. In addition, in some examples, a diagnostic methodmay be performed to determine the possible presence of a fault in thesystem based on characteristics of the bioelectrical brain signalsmonitored by sensing module 66. Although sensing module 66 isincorporated into a common outer housing 34 with stimulation generator64 and processor 60 in FIG. 3, in other examples, sensing module 66 isin a separate outer housing from outer housing 34 of IMD 16 andcommunicates with processor 60 via wired or wireless communicationtechniques.

Processor 60 may also be configured to control stimulation generator 64to deliver stimulation to diagnose a potential system fault and tofurther control sensing module 66 to sense a signal at the same time as,and/or sometime thereafter, to determine whether an evoked response wasdetermined based on the delivered stimulation, and if so, whether it wasdegraded (e.g., had a reduced amplitude, duration, an unexpectedmorphology, etc.).

Processor 60 may, in some cases, receive a signal from sensing module68, associate a portion of that sensed signal with a stimulation signaldelivered by stimulation generator 64, retrieve a template and comparethe portion of the sensed signal to the template. The result of thecomparison may then be used to determine whether a fault may exist in acomponent in the system. Instructions for controlling these functions ofprocessor 60 may be stored with memory 62 as operating instructions 77.In some examples, the sensed evoked potentials may be stored withinmemory 62 as EPs 76A. These sensed evoked potentials may be transferredto an external device such as programmer 14 via telemetry module 70 foranalysis by programmer 14 or some other external device to determinewhether a fault may exist in the system.

In other examples, processor 60 may control sensing module 66 to sense asignal that may contain an artifact introduced based on some activityin, or a signal introduced into, another portion of the patient's bodyremote from the sensing location (e.g. remote from a location in thebrain.) For instance, processor 60 may control sensing module 66 tosense a signal during, and/or sometime just after, the patient undergoesmovement. Such movement may also move, or put strain on, a systemcomponent. As one example, turning or nodding of the patient's head mayresult in stresses and/or motion placed on a lead 20A, 20B and/or a leadextension that extends along the side of a patient's neck. As anotherexample, a clinician may perform “tapping” on the can of IMD 16 tointroduce a vibrational signal into the patient's body. As anotherexample, the beating of the patient's heart likewise introduces a signalinto the patient's body.

In any of the foregoing cases, processor 60 may control sensing module66 to sense a signal at the same time, and/or a predetermined time delayafter, motion, vibrational, or other types of signals are introducedinto the body. Whether, and how, artifacts from these introduced signalsare present in the sensed signal may be useful in diagnosing a potentialsystem fault. Processor 60 may control sensing module 66 based on acommand received from programmer 14 via telemetry module 70, forinstance, or from another signal. In some cases, a clinician may “tap”on the can of IMD 16 to communicate that processor should initiatesensing by sensing module 86. For instance, the clinician may tap on thecan of IMD 16 while, or just before, the patient is instructed to turnhis/her head. Such tapping may be detected by an on-board sensor such asan accelerometer (not shown), which triggers processor 60 to initiatethe sensing operation via sensing module 66. The sensed signal or aportion thereof may be associated with the signal being introduced atthe second location (i.e., the motion artifact being introduced at thepoint of the motion, which is in the patient's neck region.

Telemetry module 70 supports wireless communication between IMD 16 andan external programmer 14 or another computing device under the controlof processor 60. Processor 60 of IMD 16 may receive, as updates toprograms, values for various stimulation parameters such as amplitudeand electrode combination, from programmer 14 via telemetry module 70.The updates to the therapy programs may be stored within therapyprograms 74 portion of memory 62. Telemetry module 70 in IMD 16, as wellas telemetry modules in other devices and systems described herein, suchas programmer 14, may accomplish communication by RF communicationtechniques. In addition, telemetry module 70 may communicate withexternal medical device programmer 14 via proximal inductive interactionof IMD 16 with programmer 14. Accordingly, telemetry module 70 may sendinformation to external programmer 14 on a continuous basis, at periodicintervals, or upon request from IMD 16 or programmer 14. For example,processor 60 may transmit brain state information 76 to programmer 14via telemetry module 70.

Power source 72 is configured to deliver operating power to variouscomponents of IMD 16. Power source 72 may include, for example, a smallrechargeable or non-rechargeable battery and a power generation circuitto produce the operating power. Recharging may be accomplished throughproximal inductive interaction between an external charger and aninductive charging coil within IMD 16. In some examples, powerrequirements may be small enough to allow IMD 16 to utilize patientmotion and implement a kinetic energy-scavenging device to tricklecharge a rechargeable battery. In other examples, traditional batteriesmay be used for a limited period of time.

FIG. 4 is a functional block diagram illustrating components of anexample medical device programmer 14 (FIG. 1). Programmer 14 includesprocessor 80, memory 82, telemetry module 84, user interface 86, andpower source 88. Processor 80 controls user interface 86 and telemetrymodule 84, and stores and retrieves information and instructions to andfrom memory 82. Programmer 14 may be configured for use as a clinicianprogrammer or a patient programmer. Processor 80 may comprise anycombination of one or more processors including one or moremicroprocessors, DSPs, ASICs, FPGAs, or other equivalent integrated ordiscrete logic circuitry. Accordingly, processor 80 may include anysuitable structure, whether in hardware, software, firmware, or anycombination thereof, to perform the functions ascribed herein toprocessor 80. Processor 80 may operate in conjunction with processor 60to perform analysis on an associated portion of a sensed signal or mayoperate alone to perform such analysis to determine whether a systemfault has occurred.

A user, such as a clinician or patient 12, may interact with programmer14 through user interface 86. User interface 86 includes a display (notshown), such as a LCD or LED display or other type of screen, to presentinformation related to the therapy, such as information related tobioelectrical signals sensed via a plurality of sense electrodecombinations in response to the delivery of stimulation to brain 28. Inaddition, user interface 86 may include an input mechanism to receiveinput from the user. The input mechanisms may include, for example,buttons, a keypad (e.g., an alphanumeric keypad), a peripheral pointingdevice or another input mechanism that allows the user to navigatethrough user interfaces presented by processor 80 of programmer 14 andprovide input.

As discussed above, if programmer 14 includes buttons and a keypad, thebuttons may be dedicated to performing a certain function, or thebuttons and the keypad may be soft keys that change function dependingupon the section of the user interface currently viewed by the user. Inaddition, or instead, the screen (not shown) of programmer 14 may be atouch screen that allows the user to provide input directly to the userinterface shown on the display. The user may use a stylus or theirfinger to provide input to the display. In other examples, userinterface 86 also includes audio circuitry for providing audibleinstructions or sounds to patient 12 and/or receiving voice commandsfrom patient 12, which may be useful if patient 12 has limited motorfunctions. Patient 12, a clinician or another user may also interactwith programmer 14 to manually select therapy programs, generate newtherapy programs, modify therapy programs through individual or globaladjustments, and transmit the new programs to IMD 16.

In one example, as user may provide input via one or more of the inputmechanisms to indicate that some signal is being introduced into thesystem. For instance, the user may push a button, speak a command,perform some tactile function on a pressure sensitive screen, and so on,to communicate that the patient has started undergoing motion. Thisinformation may be communicated to IMD 16 so that processor 60 maycontrol sensing module 66 to sense corresponding signals. Thecorresponding signals may be temporally associated with the introducedsignals and then associated portions of the sensed signals may beanalyzed (e.g., by processor 60 and/or processor 80) to determinewhether they indicate that a potential failure or component fault mayexist within the system. In some examples, at least some of the controlof stimulation delivery by IMD 16 may be implemented by processor 80 ofprogrammer 14. For example, in some examples, processor 80 may controlstimulation generator 64 of IMD 16 to generate and deliver electricalstimulation to a plurality of areas of AN of brain 28 and may furthercontrol sensing module 66 to sense a bioelectrical brain signal withinbrain 28 that is indicative of the brain activity level within the HC ofbrain 28 and that may further be used to determine whether a faultexists within the system. In other examples, stimulation may bedelivered to the HC and sensed in the AN, or stimulation may bedelivered to any first location that is functionally connected to asecond location such that stimulation at the first location will evoke aresponse in the second location.

Memory 82 may include instructions for operating user interface 86, andtelemetry module 84, and for managing power source 88. Memory 82 mayalso store any therapy data retrieved from IMD 16, such as, but notlimited to, brain activity information.

Operating instructions may guide general operation of programmer 14under control of processor 80 and may comprise instructions to controlhow signals that are sensed from electrodes 24, 26 of IMD 16 areprocessed to determine fault detection. In some example, processor 80 ofprogrammer 14, alone or with processor 60 of IMD 16, may perform thesteps to perform the methods disclosed herein, such that these methodsmay be partially, or fully, automated. Thus, while in some cases,analysis of fault data may be performed in whole or in part, by aclinician, in other cases, such analysis may be performed according totechniques described herein in whole or in part may one or moreprocessors including processor 80 of programmer 14.

Memory 82 may store evoked potentials resulting from sensing by IMD 16the result of delivered stimulation, and may include informationpertaining to whether or not characteristics of expected evokedpotentials (or evoked responses) are present within the sensed signals.

The evoked potentials could be LFP signals, or any other type of signalsthat indicate the response to stimulation. This information may furtherinclude motion date sensed by IMD 16 which is data that may result fromsensing during periods of patient motion and may include informationpertaining to whether a motion artifact is reflected in the sensedsignal. Such information may further include data pertaining to when andwhat types of motion were associated with the sensed data. Otherinformation stored by memory 82 may include ECG data that may be sensedby IMD 16 to determine whether an ECG artifact is present in a sensedsignal, such as a sensed brain signal. Other artifact data may be storedinstead of, or in addition to, ECG artifact data such as data related totapping on the housing of the IMD 16.

Memory 82 may also store baselines which provide baseline evokedresponse signals and/or baseline signals sensed during motion. Otherbaseline signals may include baseline signals when no stimulation ispresent. Different signals may be provided for different anatomicalstructures in some cases and may be indicative of those signals expectedto be received for the corresponding structures in the absence of anevoked potential signal.

These signals may be compared, for instance, to sensed signals todetermine whether in the absence of an evoked potential signal anexpected signal is being received on a particular electrode combination.In some cases, different baseline signals may be provided for differentelectrode combinations, as may be obtained just after implantation of alead when it is known that no fault is present in the system. Suchbaseline signals may be used to determine whether a lead or electrodeshifted position sometime after the baseline signals was acquired. Thevarious baseline signals may be useful in further performing faultisolation using techniques described herein.

Memory 82 may also store templates and thresholds that are of a typesimilar to those discussed above in regards to templates and thresholds76D. Thus, memory 82 may store some or all of the information shown inFIG. 3 in reference to memory 62 instead of, or in addition to, memory62 of IMD 16 storing this information.

Memory 82 may include any volatile or nonvolatile memory, such as RAM,ROM, EEPROM or flash memory. Memory 82 may also include a removablememory portion that may be used to provide memory updates or increasesin memory capacities. A removable memory may also allow sensitivepatient data to be removed before programmer 14 is used by a differentpatient.

Wireless telemetry in programmer 14 may be accomplished by RFcommunication or proximal inductive interaction of external programmer14 with IMD 16. This wireless communication is possible through the useof telemetry module 84. Accordingly, telemetry module 84 may be similarto the telemetry module contained within IMD 16. In alternativeexamples, programmer 14 may be capable of infrared communication ordirect communication through a wired connection. In this manner, otherexternal devices may be capable of communicating with programmer 14without needing to establish a secure wireless connection.

Power source 88 is configured to deliver operating power to thecomponents of programmer 14. Power source 88 may include a battery and apower generation circuit to produce the operating power. In someexamples, the battery may be rechargeable to allow extended operation.Recharging may be accomplished by electrically coupling power source 88to a cradle or plug that is connected to an alternating current (AC)outlet. In addition, recharging may be accomplished through proximalinductive interaction between an external charger and an inductivecharging coil within programmer 14. In other examples, traditionalbatteries (e.g., nickel cadmium or lithium ion batteries) may be used.In addition, programmer 14 may be directly coupled to an alternatingcurrent outlet to operate. Power source 88 may include circuitry tomonitor power remaining within a battery. In this manner, user interface86 may provide a current battery level indicator or low battery levelindicator when the battery needs to be replaced or recharged. In somecases, power source 88 may be capable of estimating the remaining timeof operation using the current battery.

FIG. 5 is a conceptual diagram illustrating example regions of brain 28of patient 12 and, in particular, regions of brain 28 included in theCircuit of Papez (also referred to as the Papez Circuit). The regions ofthe brain 28 within the Circuit of Papez are believed to be involved inthe generation and spread of seizure activity. The Circuit of Papez isone of the major pathways of the limbic system, and the regions of brain28 within the Circuit of Papez includes the AN, internal capsule,cingulate (labeled as the cingulate cortex in FIG. 5), HC, fornix,entorhinal cortex, mammillary bodies, and mammillothalamic tract (MMT).The regions of brain 28 within the Circuit of Papez may be considered tobe functionally related (also referred to herein as functionallyconnected), such that activity within one part of the Circuit of Papezmay affect activity within another part of the Circuit of Papez. In thisway, the delivery of stimulation to one region (e.g., the AN) of theCircuit of Papez may be used to evoke a response, and to affect thebrain activity level, within another region of the Circuit of Papez(e.g., the HC). In some examples, electrodes 24, 26 are implanted todeliver electrical stimulation therapy generated via stimulationgenerator 64 (FIG. 3) to and/or monitor bioelectrical brain signalswithin one or more regions of the brain in the Circuit of Papez, suchas, e.g., the AN, the internal capsule, the cingulate, the fornix, themammillary bodies, the mammillothalamic tract, and/or HC. In someexamples, a disorder of patient 12 may be effectively managed bycontrolling or influence the brain activity level within one or moreregions of the Circuit of Papez. For example, with respect to seizuredisorders, therapy may be delivered from IMD 16 to regions within theCircuit of Papez to suppress brain activity (also referred to ascortical activity) within regions of the Circuit of Papez, such as,e.g., the HC. Suppression of brain activity within the HC via therapymay reduce the likelihood of a seizure by patient 12. As anotherexample, for treatment of Alzheimer's disease, therapy may be deliveredfrom IMD 16 to regions within the Circuit of Papez to increase corticalactivity within the regions of the Circuit of Papez, such as, e.g., theHC. Increasing brain activity within the HC via therapy may reducesymptoms of Alzheimer's disease, such as memory loss. The delivery ofstimulation to the AN of brain 28 may be useful for managing a seizuredisorder because the AN is a central site of the Circuit of Papez, and,as a result, stimulating the AN may help target a plurality of seizurefoci that may be present in the Circuit of Papez even if the seizurefocus is not in the AN. Such a relationship may help minimize the burdenon a clinician in identifying a useful target stimulation site bylocating the exact seizure focus. This can be referred to as a remotestimulation approach. Moreover, stimulating in the AN can be lessinvasive to the patient because the leads can be relatively easilyimplanted in the AN compared to, e.g., the HC, although leads can beimplanted in the HC as well in some examples.

For some patients, the HC of brain 28 may be a seizure focus.Accordingly, for at least some of those patients, reducing a brainactivity level within the HC may be desirable for managing a seizuredisorder. The reduced brain activity level within the HC may helpmitigate symptoms of the seizure disorder, such as by loweringlikelihood of an occurrence of a seizure, reducing the severity orduration of seizures, and/or reducing the frequency of seizures.Stimulation (or another type of therapy) may be delivered directly tothe AN rather than directly to the HC for various reasons, such as toreduce invasiveness of the therapy system.

The level of a functional connection between the AN and the HC may becharacterized by the effect of stimulation delivery on an area of the ANon the brain activity level within the HC. As illustrated in FIG. 5,regions within the Circuit of Papez may be connected to one another vianeurological pathways such that activity within one region of brain 28may affect activity within another region of brain 28. As such,electrical stimulation delivered from IMD 16 to a particular region ofthe Circuit of Papez may influence brain signals, or evoke a response,in one or more other regions of the Circuit of Papez. Due to, forexample, the neural pathways between the different parts of brain 28,different areas within a first region (e.g., the AN or the HC) of theCircuit of Papez may have a functional connection to a second region(e.g., the HC or AN) of the Circuit of Papez, such that the effect ofthe stimulation delivery to the first region may evoke a response to thesecond region. This response may provide a therapeutic benefit, asdescribed in commonly-assigned U.S. Pat. No. 8,706,181 entitled “TargetTherapy Delivery Site Selection”, which is assigned to the applicant ofthe present disclosure and which is incorporated herein by reference inits entirety. Alternatively or additionally, the presence of thisfunctional connection may be used to diagnose system faults, asdescribed herein.

According to one technique, a relatively low frequency stimulation(e.g., 2 Hz-10 Hz) is delivered to an area of the AN of brain 28 and aresponse may be evoked within the HC. Assuming that the sensing circuit,including the sensing pathway comprising a lead, any lead extension, andthe electrodes used for sensing, are properly functioning, a local fieldpotential sensed within the HC may detect the evoked response. Asdiscussed above, this local field potential sensed immediately followingthe delivery of stimulation to the area of AN can be referred to as theevoked potential (e.g., characterized by the peak amplitude of a sensedbioelectrical brain signal), which results when the brain activity inthe thalamus propagates to the HC. Thus, the evoked potential is anexcitatory response of the HC to the stimulation delivered to thethalamus; the relatively low frequency stimulation generates a spike inbrain activity in the HC. However, if such a response is not present inthe sensed signal, or it is present but in a degraded fashion, it may bedetermined that a fault may have occurred either in the stimulationcircuit, including the stimulation pathway such that no stimulation (orstimulation at reduced levels) was delivered to the AN or a fault mayexist within the sensing circuit, including the sensing pathway.

The foregoing is merely one example of delivering stimulation to the ANand sensing in the HC, and instead stimulation may be delivered to theHC and sensed in the AN, or delivered at some other site in the circuitof Papez or in some other circuit in the brain and sensed at a locationfunctionally coupled to the location of stimulation delivery.

In some examples, the evoked response (such as the response in the HC ofbrain 28 of patient 12) can be determined based on at least onecharacteristic of a sensed bioelectrical brain signal which can be afrequency domain characteristic or a time domain characteristic of thebioelectrical brain signal. Examples of a time domain characteristicinclude, but are not limited to, a mean, median, instantaneous, peak orlowest amplitude of the bioelectrical brain signal within apredetermined period of time. Examples of a frequency domaincharacteristic include, but are not limited to, a power level in one ormore frequency bands of a bioelectrical brain signal (e.g., sensedwithin the AN or HC) sensed over a predetermined period of time or aratio of power levels in at least two frequency bands of thebioelectrical brain signal. In some currently proposed techniques, atarget therapeutic stimulation site within the thalamus is selected tobe the area of the thalamus that resulted in the greatest evokedpotential in the HC (e.g., the brain signal with the greatest averagepeak amplitude for a time period following the delivery of thestimulation to the thalamus). It is believed that the area of thethalamus that is associated with the greatest evoked potential has thestrongest functional connection to the HC, such that delivery ofstimulation to the area of the thalamus may provide efficaciousstimulation therapy for managing the seizure disorder of patient 12.Such a site selection may also prove beneficial when analyzing faults,since stimulation at this site should, barring the existence of faults,provide a response that is readily apparent in the sensed signal.Example methods for selecting a site for stimulation in one area forsensing in another area of the patient's body are disclosed in U.S. Pat.No. 8,706,181 referenced above and incorporated herein by reference.

FIG. 6 is a flow diagram illustrating an example technique for usingsensed signals to determine whether a potential fault may exist withinthe system. A signal may be sensed at a first location (100). The signalmay be an LFP, an EEG and ECoG, a microelectrode recording signal (MER),or some other type of signal sensed by the system. The first locationmay be a location on the patient's body, such as within brain tissue, onthe cortex, within the subgaleal space, on the scalp, or some otherlocation within the body.

A signal may be introduced at a second location different from the firstlocation (102). The introduction of the signal may involve a signalintroduced by motion of the patient's body. For instance, the patientmay undergo some movement such as head turning, arm swinging, movementof the torso, or some other movement that is performed by the patient.Such motion may be performed at the direction of a clinician for thepurpose of diagnosing a potential fault, or may be motion undertaken bythe patient for another purpose. In one example, the motion may bephysiological activity that is not consciously controlled by thepatient, such as beating of the patient's heart. In still anotherexample, the signal is an acoustic signal resulting from tapping on thehousing of IMD 16 or otherwise manipulating a component of the systemsuch as palpating, moving, pushing on, and so on, a lead or leadextension that is under the skin of the patient.

In any event, the second location of the body at which the signal isintroduced may be a portion of the body undergoing or affected by themotion. The second location may be the neck, the arms, the torso, thebeating heart, and so. The signal may be a vibrational signal, anacoustic signal (e.g., introduced by a heartbeat), a pressure signal, asignal resulting from stresses and strains placed on a lead, leadextension, or some other component in the system, or some other signalintroduced at the second location. The signal may be introduced before,during, or after sensing at the first location has commenced. A portionof the sensed signal may be associated with the introduced signal (104).

This may involve determining a portion of the sensed signal that iscontemporaneous with the introduced signal. For instance, a portion ofthe sensed signal that was sensed during, a predetermined time within,or a predetermined time after, the introduction of the signal at thesecond location may be associated with the introduced signal. In somecases, this may involve providing, by a user of the system, someindication that motion has been commenced, is underway, or has juststopped. Such an indication may involve providing input to the systemthat can be used by the system to timestamp or otherwise annotate thesensed signal. Input may be provided via a patient or clinicianprogrammer, tapping on a housing of the IMD 16 in a way that can besensed by an on-board sensor such as an accelerometer, or using someother input mechanism.

In some cases, the association may be performed automatically. Forinstance, when a portion of the sensed signal is determined to have aparticular characteristic that is known to originate outside the firstlocation, the system may automatically perform the association. Such acharacteristic may be a signal amplitude that is above some thresholdthat is known to be expected in a signal sensed from brain tissue. Asother examples, the characteristic may be a frequency, a waveformmorphology, a pattern or timing of the reoccurrence of a characteristicthat is not otherwise expected to manifest itself in the tissue.Detection of these types of characteristics may cause the system toautomatically associate the sensed signal with a signal introduced at asecond location.

The portion of sensed signal that has been associated with theintroduced signal may be identified in various ways. In some cases, thesensed signal may be stored within a memory of IMD 16 or programmer 14,with the associated portion of this signal being stored along withtimestamps or other annotations. In some cases, only the associatedportions may be stored, with other portions of the signal beingdiscarded or stored in another location of memory. The associatedportions may be stored along with some indication that describes theintroduced signal. The description may involve a recording of theintroduced signal recorded at the second location (e.g., via electrodeson the patient's body), a description of the signal provided by a user,and so on. The sensed signal, or an associated portion thereof, which isstored for later analysis may include “raw” data such as data that isconverted from an analog to a digital signal but which is otherwiseunprocessed, or a signal that has undergone A/D conversion and hasfurther undergone some other type of processing, such as some filteringto remove some unwanted noise. Such signal data may be stored withinmemory 62 of IMD 16 and/or memory 82 of programmer 14.

After a portion of the sensed signal is associated with the introducedsignal, it may be determined if the associated portion is indicative ofa fault (106). Such analysis may involve comparing a time domain featureof the signal, such as an amplitude, waveform shape, the rhythmic natureof the signal (e.g., whether spikes or other characteristics are knownto be occurring at a frequency of the introduced signal), and so on. Insome cases, a signal may be compared to one or more thresholds todetermine whether the signal is outside of expected ranges. Forinstance, a sensed signal sensed within brain tissue that has anamplitude within a millivolt range may be known to be outside ofexpected physiological signals, since physiological signals originatingin brain tissue are expected to be in a microvolt range. This mayindicate the presence of a fault. As another example, spikes in thesignal occurring at a regular interval that is known to correspond to afrequency of patient motion, frequency of tapping on the “can” of IMD16, or the frequency of the patient's beating heart may be determined tobe the result of an introduced signal. Typically such signals would notbe sensed within brain tissue by leads implanted within the brain.However, the occurrence of faults such as leakage pathways that allowfor fluid ingress at connection points in the system may allow suchsignals to be sensed by leads, obscuring any signal that would otherwisebe sensed within the tissue. The detection of this type of signal may bean indication that a fault may have occurred within the system. Baselinesignals may be used to determine whether a fault likely exists. As anexample, an associated portion of the sensed signal may be compared to abaseline signal that reflects how the introduced signal is expected tolook in a system without faults. Such a baseline signal may reflect thecomplete absence of any artifact in the signal if, when no fault ispresent, the motion is not expected to affect the sensed signal. Inanother example, the baseline signal may reflect low levels of theartifact. This baseline signal may be obtained when motion is introducedinto the system when a fault is known to be absent. In some cases, thebaseline signal may be patient-specific in that it is derived fromsignals sensed by the system currently being verified when it was knownthat the system way free of faults. In this case, a match between thesensed signal and the baseline indicates that the comparison does notpoint to the likely existence of a fault in the system.

According to another scenario, a template may reflect how the introducedsignal might be expected to look if a fault likely exists in the system.Such a template may be obtained by sensing a signal in a system known tohave a fault when the particular type of motion is introduced into thesystem. In this case, the template may, for instance, include one ormore noise “spikes” or other disturbances that are likely to occur whenthe predetermined type of motion is introduced when a fault is present.Different templates may be provided for different types of motion,different system components and configurations, and so on.

Based on any combination of techniques described herein, if it isdetermined that a sensed signal is not likely indicative of a fault(108), this information may be provided to a user, as via informationpresented on a display of a user interface of programmer 14. If, on theother hand, a sensed signal is likely indicative of a fault (110),additional analysis may be performed. In some cases, this may beaccomplished using different templates that may be developed fordifferent fault types. Such templates could be determining by recordingsignals in a system known to have the particular type of failure. As anexample, in a system known to have an IMD header block that is allowingfluid ingress, a signal may be recorded by a lead implanted in the brainwhile the patient turns their head. Any number of fault types may beused to develop respective artifact templates.

In some cases, a templates may be derived based on a combination of oneor more factors, including fault types, the system configuration (i.e.,types of components in use in the system), location of the sensing lead,the type of artifact that is introduced into the system and/or any otheraspect that would affect the template. The templates may be derived bysensing a signal in a system having a known type of fault, and furtherhaving the type of configuration, and experiencing the type of artifactthat is associated with the template.

During use, a particular set of templates may be selected for comparisonto a sensed signal based on a type of artifact being analyzed, systemconfiguration (i.e., the selected set of templates was derived using asystem similar to that in use with this patient), and so on. That set oftemplates may then be used to diagnosis the likely cause of the fault.For instance, a signal sensed while the type corresponding type ofartifact is introduced into the system may best match the templateassociated with fluid ingress between the lead and lead extension(versus other templates associated with other types of faults). Thismatch may be used to diagnose a likely problem with one the connectionbetween these two components.

Analyzing the fault (110) may be performed by processor 60 of IMD 16and/or processor 80 of programmer 14. Results of the fault analysis maybe provided to the user via a display of programmer or via some otheruser interface mechanism. In some embodiments, this may involvedisplaying the sensing signal with the introduced signal for the user toanalyze. The display may include time stamps to indicate when theartifact was introduced relative to the sensed signal. In someembodiments, multiple waveforms may be overlaid on one another toprovide a time correlation, with a first waveform representing thesensed signal at the first location and the second waveform representingthe introduction of the signal at the second location. In some cases,the fault analysis (110) may be provided automatically to a user. Thisoutcome may be provided as a “yes/no” indication as to whether a faultlikely exists within the system. Instead, the outcome could be providedquantitatively, such as using a percentage likelihood that a fault hasoccurred. In some cases, guidance as to the likely root cause of thefault may be provided. For instance, as discussed above, different typesof faults may be associated with different templates or signatures thatare stored in memory. By comparing different ones of these templates tothe sensed signal and determining whether the sensed signal matches anyof the templates, it may be determined whether a likely match occursthat indicates the type of fault. In some cases, such a match may beachieved by the comparison meeting some predetermined confidence levelas determined by the template matching algorithm in use (e.g., there isa 90% chance that a match occurred). In this manner, the system mayautomatically provide guidance as to the likely cause of the faultand/or may even recommend an action to be taken to remedy the fault.Further fault analysis (110) may be performed using information obtainedfrom various sources. For instance, as discussed above, additionalinformation may be obtained by determine if, or what type of, an evokedresponse is being sensed at a first location when stimulation isintroduced at a second location, as discussed in reference to FIG. 7.This type of information may be used to further isolate the source ofthe fault. FIG. 7 is a flow diagram illustrating sensing an evokedpotential at a first location when stimulation is delivered to a secondlocation. Specifically, a signal may be sensed at one or more firstlocations (120). Stimulation may be delivered at a second location(122). The second location may be location having some type offunctional connectivity to the first location. This second locationwhich has a functional connectivity to the first location may bedetermined using techniques discussed below.

The sensing and stimulation may occur contemporaneously with oneanother. For instance, sensing may occur throughout the entire period oftime stimulation is delivered or may commence after a delaycorresponding to the time required for the stimulation signal to travelfrom the first to the second location. In other cases, sensing maycommence prior to stimulation, and stimulation may occur only during aportion of the time of the sensing.

In any event, a portion of the sensed signal may be associated withintroduction of the signal at the second location (124). The associatedportion of the sensed signal may be that portion that would be expectedto be affected by the introduction of the stimulation at the secondlocation assuming there is some type of functional connectivity thatphysiologically exists between the first and second locations. Forinstance, this may be the time segment of the sensed signal which wassensed in the HC that is expected to exhibit an evoked response becauseof stimulation delivered to the AN of the thalamus of the patient. Thistime segment of the sensed signal may be determined based on an expecteddelay between delivery of stimulation to the AN and detection of theevoked potential in the HC, which may be a patient-specific delay.

Next, it may be determined whether a response to stimulation as may bedetermined by the associated portion of the sensed signals meetspredetermined criteria (126). This may be determined in any of the waysdiscussed herein, including comparing a time-domain or frequency-domaincharacteristic of the signal to a baseline signal, a template or athreshold. For instance, a baseline signal may be stored in memories 60and/or 80 that correspond to evoked potentials that are expected tooccur in systems that do not have any faults. This baseline signal maybe patient-specific in that it was sensed, or otherwise derived from asensed signal, during an evoked response at the first locations thatresulted from stimulation at the second location when the system wasknown to be fault free.

If the response does meet expected criteria, it may be determined that afault does not likely exist within the system (128). Otherwise, faultanalysis may be performed (130) to further identify a likely cause ofthe fault. Further fault analysis may be performed automatically or withthe help of the clinician. For instance, the associated portion of thesensed signal may be displayed on a screen of programmer 14 for viewingby a clinician who may determine a likely cause of the fault.Alternatively or additionally, the system may automatically performfurther fault analysis.

In some cases, the system may compare the associated portion of thesensed signal to multiple templates, each associated with a differenttype of fault in a similar manner to that described in regards to FIG. 6step 110. For instance, the associated portion of the sensed signal maybe compared to different templates with the closest match indicating atype of fault. Alternatively, or additionally, the portion of the sensedsignal may be compared to a baseline signal of the evoked response.

FIG. 8 is a flow diagram illustrating used of a baseline signal inanalyzing whether a system fault has occurred within the system. Theassociated portion of a sensed signal may be compared to a baselinesignal (140). The baseline signal may be indicative of an evokedresponse signals that is sensed when it was known that a fault does notexist in the system. The baseline signal may be obtained from thepatient or may instead represent a signal obtained from patientpopulation data. It may be determined whether the sensed signalindicates a fault occurred (142). For instance, if the sensed signaldeviates by a certain amount, percentage, or in certain ways (e.g., doesnot have similar waveform shape, deflection points, or other signalcharacteristics) from the baseline, it may be determined that a faulthas occurred in the system. In some examples, the way in which thesensed signal deviates from the baseline may be informative of the typeof fault. For instance, if an evoked response appears to be presentwithin the sensed signal but is degraded or does not havecharacteristics that are expected, it may be determined that somestimulation was delivered at the second location, but a fault may existwithin the sensing path to compromise the signal sensed at the firstlocation. A similar conclusion may be drawn if no evoked response issensed at all and a comparison of the associated portion of the sensedsignal to a baseline signal obtained when no stimulation is delivered atthe second location results in a favorable comparison. In this case, thesensed signal is representative of what would be expected in the absenceof stimulation, again leading to a conclusion that sensing is occurringfault free but some fault may be present in the stimulation pathway. Onthe other hand, the sensed signal may not compare favorably to eitherthe baseline signal associated with an evoked response or the baselinesignal expected when no stimulation is delivered. In this case, it maybe that the sensing path contains the fault.

If the comparison (142) results in a “match” of the associated portionof the sensed signal to a baseline, as determined be known patternmatching techniques such as those discussed above, no fault isdetermined to occur (144) as may be reported to a user via a userinterface, such as a display of programmer 14. Otherwise, if the evokedresponse is not detected or appears to deviate from the baseline in amanner that is indicative of a likely fault, the fault may be reported(146). In some instances, it may be possible to suggest a likely causeof the fault based on analysis such as discussed above.

In some cases, further fault analysis may be performed by repeating themethod of FIG. 7 but stimulating at the first location while sensing atthe second location (assuming a functional physiological relationshipexists between the first and second location that allows stimulation atthe first location to cause an evoked response at the second location.)For instance, if the first comparison in step 142 of FIG. 8 suggestedthat the fault is likely in the stimulation path, repeating the steps ofFIG. 7 but stimulating at the first location while sensing at the secondlocation may help confirm the initial fault diagnosis if the fault nowappears to be in the sensing pathway, and so on.

FIGS. 9A and 9B are examples of waveforms representing signals sensed inthe brain of a patient during a fault condition and during a non-faultcondition, respectively. In both figures, the Y axis indicates signalamplitude in millivolts, and the X axis represents time over which thesignal was sensed. The location of sensing in this example is the HC.

In FIG. 9A, the four waveforms are shown, each corresponding toelectrical stimulation delivered to the AN while sensing occurred in theHC. The portion of the waveform shown may be described as the associatedportion of the sensed signal, since it is the portion that wouldcorrespond temporally to the delivery of the stimulation. That is, theportion may be sensed substantially simultaneously with, or in somecases after a predetermined time delay of, the delivery of thestimulation.

The four waveforms 160, 162, 164, and 166 each correspond to delivery ofstimulation at the AN at a respectively different stimulation amplitudeof 6V, 5V, 4V, and 3V. In all cases, waveforms 160-166 indicate that thestimulation resulted in some type of response, but the response is notas would be expected in a system free of faults. This may be appreciatedby comparing waveforms 160-166 to the waveforms of FIG. 9B, which areevoked responses sensed within a system over a period of 64 weeks in asystem known to be free of faults. The waveforms of FIG. 9B all commencewith a rather sharp negative slope followed by a local minimum value, alocal maximum value (a small “peak”) that occurs between 20 and 40milliseconds, another local minimum value, followed by a maximum peakvalue occurring between 60 and 90 milliseconds. These characteristicsare seen to be attenuated in the four waveforms 160-164 of FIG. 9A. Infact, each of the waveforms of FIG. 9A commence with a sharppositive-going slope and a maximum peak value occurring around 10milliseconds, followed by a local minimum value occurring around 20milliseconds. The peak value occurring between 60 and 90 milliseconds isdegraded from those appearing in the waveforms of FIG. 9B.

The signals of FIG. 9A are examples of baseline signals that may berecorded from a patient in a system known to be free of faults. Thebaseline signals may be recorded over a period of time as shown in FIG.9A. The baseline signals may be averaged or processed in some othermanner to derive a single baseline that may be stored for comparison tothe associated portion of sensed signal. Comparison may includecomparing characteristics of the baseline such as timing, amplitude, andlength of inflection points, local maximum and minimum points, absolutemaximum and minimum points, slopes of certain points of the waveform andso on. Faults may be detected in some cases based on an absolute amountof deviation from the baseline, a percentage deviation, the lack ofexpected characteristics, and so on.

In some instances, as discussed above, evoked responses may be entirelylost. This may occur when an open circuit condition occurs, such as if aconductive pathway in a lead is broken, or a contact becomesdisconnected or broken. In these circumstances, the complete loss of anevoked potential may correspond to very high impedance values that canbe measured between one or more pathways in the system. The highimpedance values may be consistently present and measurable within thesystem (as when the fault is a “hard” failure that is not transient), orinstead may be intermittent as when the fault is transient. Impedancevalues may be determined to be too high, or “out-of-range” by comparingthem to baseline impedance values that are expected to occur whenimpedance values are in-range. Comparing measured impedance values tobaseline impedance values may help confirm the presence of a fault inthe system that is suspected based on the evoked response data. Theimpedance values may further help to pin-point which component in thesystem may be failing, as discussed in regards to FIGS. 10A-10D below.

FIG. 10A illustrates multiple plots of impedance values measured betweenvarious electrode pairs of an implantable 3387 model lead, which iscommercially available from Medtronic, Inc., that has been implanted inan ovine subject. This model lead includes four ring electrodes similarto what is shown as the four electrodes 24 and 26 of leads 20A and 20B,respectively, of FIG. 1. The impedance values were measured at varioustimes following lead implantation in an ovine subject. Impedance valuesare plotted against the Y axis and time is represented by the X axis. Asindicated by the key, four of the plots represent impedance measurementsbetween a respective one of the four lead electrodes (designatedelectrodes “0” through “3”) and the housing of the implantable device.These four plots are labelled, respectively, C-0 through C-3. Sixadditional plots represent impedance measurements between various pairsof electrodes on the lead and are labeled by the respective electrodedesignations. For instance, plot 0-1 represents the impedancemeasurement between lead electrodes 0-1.

In a manner similar to FIG. 10A, FIG. 10B illustrates impedance valuesbetween various electrode pairs of a system comprising a 3389 model leadcommercially available from Medtronic, Inc. which has been implanted inan ovine subject. The measurements are taken at different timesfollowing lead implantation as shown along the X axis. Impedance isplotted against the Y axis. As in FIG. 10A, four of the plots, labeledC-0 through C-3, represent impedance measurements between the four leadelectrodes “0” through “3” and the housing of the implantable device.Six additional plots represent impedance measurements between variouspairs of electrodes on the lead and are labeled by the respectiveelectrode designations.

FIGS. 10A and 10B each represent impedance data obtained when the evokedresponses, i.e, stimulation evoked motor responses (SEMRs), are stable.That is, the evoked responses that were sensed at the time of theseimpedance measurements were within a range that would be expected in asystem with no faults manifesting in either the stimulation or sensingpathways that would affect the sensing or generation of the evokedresponse. These impedance plots may be considered to represent baselineimpedance measurements that would be expected in the absence of faultsaffecting impedance.

The impedance data similar to that shown in FIGS. 10A and 10B may beused to confirm the possible presence of a fault as indicated by evokedresponses measurements. For instance, baseline impedance data for apatient may be measured at one or more times following implant when itis known a fault does not exist within the system. The data may bestored within memories 62 and/or 82. Sometime thereafter, if sensingindicates either loss or degradation of an evoked response, impedancemeasurements may be taken between various electrode pairs in the system.These impedance measurements may be compared to the previously-storedbaseline impedance data. This additional comparison between impedancevalues may be used to further confirm the presence of the fault.

In some examples, the impedance data may help pin-point the origin ofthe fault. For instance, measurements between various electrode pairscan be taken and compared against corresponding baseline measurements todetermine if the fault seems to be limited to just one pair ofelectrodes or multiple pairs. If the impedance data indicates the faultmay be limited to just one pair, the evoked response data may be used todetermine whether the fault may be associated with the stimulation orthe sensing path, as discussed above. On the other hand, if theimpedance data indicates multiple impedance values are out-of-range, anevaluation may be performed to determine what logic is in common betweenthe pathways represented by these measurements. This common logic may beidentified as the potential source of the fault. FIG. 10C illustratesmultiple plots of impedance values measured between various electrodepairs of an implantable 3389 model lead implanted in an ovine subject.As in FIGS. 10A and 10B, impedance is plotted against the Y axis andtime is represented by the X axis. The various plots correspond to thevarious electrode combinations discussed above for FIGS. 10A and 10B,including plots for C-0 through C-3 between electrodes 0-3 and thehousing, respectively, and additional plots 0-1, 0-2, 0-3, 1-2, 1-3, and2-3 for each of the six possible pairs of the electrodes on thefour-electrode lead. The impedance values were measured at various timesfollowing lead implantation in an ovine subject.

Sometime between day 29 and 35, evoked response measurements indicatedthat the evoked responses were lost (i.e., were not sensed at all, orsubstantially degraded) for those measurements involving electrodes “0”and “1”. The impedance data also shows that all of the impedance plotsinvolving these electrodes, including C-0, C-1, 0-1, 0-2, 0-3, 1-2, and1-3 indicate out-of-range impedance values. Only the impedance plots forC-2, C-3 and 2-3 illustrate in-range impedance values such as those thatcorrespond to what is shown in the baseline data for the 3389 model leadin FIG. 10B. Evoked response waveforms sensed for electrodes “2” and “3”confirm that these evoked responses remain stable, confirming that thereare likely no faults in the pathways associated with these twoelectrodes. In this manner, the impedance data confirms the possiblefault that was already indicated by the evoked response data for the “0”and “1” electrodes, and also helps confirm that there are likely nofaults associated with electrodes “2” and “3”.

The set of impedance plots depicted by FIG. 10C, which were obtained ina system known to have broken contacts for electrodes “0” and “1” may,in some cases, be stored as templates. These templates may be comparedagainst out-of-range impedance values to help pin-point the source of afault. For instance, if a set of out-of-range impedance plotssubstantially matches a stored set such as shown in FIG. 10C, it may bedetermined that contacts “0” and “1” are broken. Any number of sets oftemplates corresponding to known system faults may be stored in thismanner for use in further analyzing the source of a system fault.

FIG. 10D illustrates another set of impedance value plots measuredbetween various electrode pairs of an implantable 3389 model leadimplanted in an ovine subject. The same set of electrode pairs arerepresented by the various plots as those shown in FIGS. 10A-10C, and asin FIGS. 10A and 10C, the Y axis indicating impedance values and the Xaxis representing time.

The data shown in FIG. 10D is associated with an unknown problem. Whatis important to note is the correspondence between out-of-rangeimpedance values and the loss of evoked potential. For instance, aroundday 6, impedance values for many of the electrode pairs begin to climb.At day 14 and after day 28, the evoked response signal is lost orsubstantially degraded for a subset of those electrode combinationsdemonstrating out-of-range impedance values. While impedance values forthe various combinations of electrodes that are associated without-of-range impedance begin to drop between days 14 and 28, the valuesnever-the-less remain out-of-range as indicated by a comparison betweenthe data of FIGS. 10B and 10D. The loss of the evoked response signalafter day 28 for some of the electrode combinations confirms thepossibility of the fault. Thus, by using both evoked response data andthe impedance data, the possibility of a failure can be furtherconfirmed and, in some cases, more accurately pin-pointed.

In some cases, out-of-range impedance values may be consistently sensedwithin the system, indicating the fault is manifesting itself most, orall, of the time within the system. In other cases, the out-of-rangeimpedance values may be obtained only intermittently. This may be thecase, for instance, if a lead or lead extension has a fracture thatresults in an open circuit upon patient motion. For instance, a leadextension that may be tunneled under the skin of the patient's neck,running from the proximal end of the lead located on the patient's headto an implantable device implanted in the patient's torso. A fracture inthe lead extension may only present a high-impedance pathway when thepatient turns his or her head, causing the fracture to separate.

An intermittent fault of the type resulting in out-of-range impedancemeasurements may be diagnosed by obtaining impedance measurementssubstantially real-time. A succession or sequence of impedance valuesmay be obtained over time as, for instance, the patient is undergoingmotion. The impedance measurements may be time-correlated with thepatient's motion so that opens and shorts occurring as the patient moveswill be reflected in the time-sequence of measurements obtained for aparticular circuit pathway (e.g., the pathway including multipleelectrodes and the conductors coupled to these electrodes.)

As a particular example of the foregoing, a user such as a clinician orpatient may provide input such as by interacting with a user interface86 of a programmer 14 (FIG. 4) or some other external device and/or bytapping on the IMD 34 in a way that may be detected by an on-boardaccelerometer. Such user input may be provided at a time thatcorresponds with, or is time-correlated to, the turning of a patient'shead or some other movement. Such input may cause a timestamp or someother marker to be stored in memory to indicate time(s) of patientmotion. This timestamp or marker data may be introduced into a stream ofimpedance values by a system clock (e.g., a clock of IMD 34 and/or aclock of programmer 14) or otherwise stored with the data in memory 62.In this way, it may be possible to determine which portion of a sensedsignal corresponds to a time at which the motion is occurring.

A sequence of impedance values that includes marker or timestampinformation indicating which values correspond with motion may be storedin memory 62 of IMD 34, stored within memory 82 of programmer 14, storedin some other external device (e.g., a clinician workstation a cellphone or PDA, or some other device), and/or uploaded to “the cloud” forstorage on a central database. This data may further be used to generateinformation that may be presented to a clinician. For instance, adisplay on a screen of programmer 14 may be used to illustrate in agraphical or other format the variation in impedance over time. Thisgraphical or other display could be annotated to indicate the times atwhich patient movement occurred (e.g., the times of head movement) sothat the clinician can determine whether a likely short or open isoccurring intermittently with some type of movement.

A display of the type discussed above may also be correlated (e.g., on acommon time axis) with a graphical representation of the patient'smovement (e.g., a representation of a head performing rotationalmovements at the same frequency as was performed by the patient) alongwith a rolling window displaying the stream of impedance data so that aclinician can determine a likely rotational position of a patient's headat the time of an intermittent short of open, and thereby aid in thediagnosis of the system fault.

In a manner similar to the foregoing, other signals may reflect thepresence of patient motion. For instance, LFPs, ECoGs, EEGs or someother physiological sensed signals may be acquired at a time of patientmotion. In a manner similar to that discussed above in regards toimpedance signals, timestamps or other markers may be stored along witha portion of the physiological sensed signal that correspondstemporarily with the motion. These portions of the signals that weresensed during, or substantially during, the occurrence of the patientmotion can then be analyzed to determine whether a motion artifact isevident in the signal. Motion artifacts may be manifested as short,high-amplitude, spikes that are non-physiological in nature. Suchsignals may be characterized as non-physiological because of theirconsistent non-physiological frequency (e.g., corresponding to frequencyof patient motion rather than any frequency naturally occurring withinthe physiological signal). This may allow the user to more readilydetermine the likelihood of a fault, and may provide furtherconfirmation of a motion-induced open or short in the system. Forinstance, a motion artifact may not be evident in an LFP signal sensedwithin a system that does not have a fault. Conversely, if a system doeshave a fracture or other intermittent failure, an intermittent leakagepath may result that affects noise and/or stimulation rejection,interfering with the sensed signal. This is as shown in FIGS. 11A and11B.

FIG. 11A is a conceptual diagram illustrating the frequency content ofan LFP signal over time in a system free of a fault. The signal of thisexample is sensed in the STN of a patient.

The frequency content of the signal is indicated along the Y axis andtime is depicted on the X axis. The diagram of FIG. 11A represents ascenario with no system fault and no motion present. The LFP is as wouldbe expected for the patient, and may be considered a baseline LFP signalfor a lead implanted in the STN.

FIG. 11B is a conceptual diagram similar to FIG. 11A illustrating thefrequency content of an LFP signal over time in a system that includesan intermittent fault in the lead sensing an LFP signal in the STN.During a portion of the time represented by FIG. 11B, the patient, whichis an ovine subject, is moving its head. This causes a break thatimpacts rejection of the stimulation signal which is being delivered tothe patient at the same time as the signal is being sensed. As a resultof the compromised stimulation signal rejection during the period ofmotion (e.g., extending between about 30 and 130 seconds), the sensedLFP signal contains content in frequencies that are harmonics andsub-harmonics of the delivered stimulation. This is shown in FIG. 11B assignal content at about 20 Hz, 40 Hz, 60 Hz, 80 Hz, 100 Hz, and so on,in regular increments.

In the example of FIG. 11B, the artifact introduced into the signal is astimulation artifact resulting from degraded stimulation rejection. Themotion of the patient resulted in the manifestation of the fault, whichcaused a leakage path resulting in this stimulation degradation. Inother cases, the motion itself may cause a motion artifact. This may beapparent as frequency content in the sensed signal that corresponds tothe frequency of the patient motion. For instance, the patient may bedirected to turn his or head at a frequency of about once every second.This may cause extraneous frequency content of about 1 Hz to appear inthe signal.

Regardless of the artifact source, the presence of this artifact in thesensed signal may be used to detect the intermittent failure. This typeof signal analysis may uncover a potential fault even in scenarioswherein impedance measurements of the type discussed herein appearnormal. For instance, if fracture is small enough, impedancemeasurements in the affected pathway may not be out-of-range but LFP orother physiological signals sensed with the pathway may still manifestthe type of extraneous content depicted in FIG. 11B, thus leading toearly diagnosis of the fault.

FIG. 12 is a flow diagram illustrating a method of diagnosing a fault bydetecting an artifact in the signal. A signal may be sensed at a firstlocation (150). For instance, the first location may be within a brainof a patient, such as an STN or any other location. A signal may beintroduced at a second location that may be different from the firstlocation (152). This signal may comprise stimulation introduced at asecond location within the STN or may be delivered to an entirelydifferent structure of the patient's brain. The second location may, ormay not, be functionally connected to, or associated with, the firstlocation. In other cases, the introduced signal may be a motion artifactcaused by the patient turning his or her head. In yet another example,an artifact may correspond to the beating of a patient's heart ortapping on the housing of a medical device at a predetermined frequency.

A portion of the sensed signal may be associated with the introducedsignal (154). For instance, a timestamp or other marker may be stored,or otherwise associated, with the portion of the sensed signal that wassensed while the signal was introduced at the second location. If thesignal was introduced at the second location for the entire time thesignal was sensed at the first location, the entirety of the sensedsignal may be associated with the introduced signal.

It may be determined whether an artifact is present in this associatedportion of the sensed signal (156). If not (158), the sensed signal maynot indicate the presence of the fault. If an artifact is present,however, fault analysis may be performed (160). This may involvecollecting and analyzing other data such as evoked response(s) andimpedance measurement according to techniques described herein. It mayfurther involve obtaining patient feedback, such as whether they areexperiencing loss of therapy at one or more times (e.g., during periodsof patient motion) and so on. As discussed above, in some cases LFP,ECoG, EEG, or other physiological signals may be sensed in real-time orsubstantially in real-time. Such data may be time-stamped or otherwisetagged to indicate which signal samples correspond to patient motion. Anexample of such data is shown in FIG. 13.

FIG. 13 is a signal diagram of two LFP recordings obtained from the ANand HC of an ovine subject. The signal amplitude (in volts) is plottedalong the Y axis while the X axis represents time (in seconds). The topwaveform is that obtained from the AN and reflects the movement artifactoccurring when the patient intermittently moved its head, as isrepresented by one or more signal “spikes” 160A-160D in this signal. Thebottom waveform was obtained from the HC of the subject and does notinclude the signal spikes.

In some examples, the type of LFP recordings shown in FIG. 13 may beobtained by collecting the signal data in real-time or substantially inreal-time and storing this data to memories 62 and/or 82 using datastreaming, for instance. In some cases, this data may include one ormore time-stamps or markers. For instance, a user may be allowed toprovide input via a user interface of programmer 14 indicating when thepatient moves. This input may result in markers or time-stamps beingstored with the data to indicate which data sample(s) correspond with,(or were collected at substantially the same time as), the movement.Alternatively, the movement indication provided by a user may beassociated with a time indication and may be stored separately and laterused by the system to annotate the data. As still another example, amovement indication may be provided automatically by an accelerometer,gyroscope, or other movement sensor worn by the patient.

In examples, a display may be provided to a user via user interface 86of programmer 14 or some other user interface that of the LFP data. AnLFP waveform may include markers indicating time of motion.Alternatively, a separate waveform illustrating time of motion may bedisplayed with, or may overlay, the LFP waveform in a time-correlatedmanner that allows a user to readily determine which portion of the LFPsignal corresponds with the motion. In this manner, the user maydetermine if the motion is causing a motion artifacts, as may be thecase if an intermittent fault has occurred in the system. Such faultsmay not even be apparent from impedance measurements. Therefore, themotion data may help with the early detection of such things as hairlinefractures in conductors of a lead or lead extension, a fluid leakagepathway resulting from incomplete sealing or an insulation breach, orsome other fault that would otherwise be impossible or difficult todetect with just impedance measurements.

While FIG. 13 illustrates an LFP signal that contains motion artifactsresulting from the turning of a head, other types of motion might causethese artifacts, such as the swinging or other movement of arms and/orhands, twisting of a torso, movement in leads and/or feet, bending atthe waist, or the movement (beating) of the patient's heart. Asdiscussed above, the times and/or frequency of this movement data may bestored in memory and used to annotate the sensed LFP data or otherwiseused to indicate to the user which portions of the sensed datacorresponds with the movement. In some cases, the frequency of themovement (e.g., frequency of the beating heart) will be enough todetermine whether the LFP signal is being affected by that movement,since the frequency of the signal artifacts will correspondsubstantially to that of the movement.

The above example illustrates an artifact in a signal obtained from anelectrode in the AN of the patient's brain. Signals obtained from otherlocations in the patient's brain, such as the STN, may likewise containsimilar motion artifacts. Moreover, which the signals of FIG. 13 are LFPsignals, other physiological signals may contain motion artifacts andmay be used in a similar manner to determine faults. For instance, EEG,ECoG, and other physiological signals may be used in a similar manner.

As described herein, using some type of introduced signal, such as amotion artifact, a cardiac waveform artifact, or stimulation that can besensed at a remote location may be used in any combination to helpdetect a fault in a medical device system. This information may helpdetect a fault even before impedance measurement may indicate that afault has occurred and may be particularly useful in detectingintermittent faults. Impedance measurements may be used to furtherconfirm a fault exists. In particular, impedance measurements obtainedin real-time during patient motion may help detect intermittent faultsthat are only apparent during such motion.

Another way to uncover a potential fault is determine whether theefficacy of a particular parameter set used to deliver therapy haschanged. For instance, if a fault is occurring that is resulting in anincrease of impedance of a lead or lead extension in a system that usedvoltage-controlled stimulation, the therapy received by the patient at aparticular stimulation amplitude may no longer be efficacious. Toreceive the same level of therapy as was previously experienced prior tothe fault may require an increase in the stimulation amplitude. This canbe determined by periodically performing therapy titration and obtainingpatient feedback, as described in reference to FIG. 14.

FIG. 14 is a flow diagram illustrating therapy titration performed todetermine whether a change in therapy efficacy may have occurred. A setof stimulation parameters may be selected for delivering therapy to thepatient (170). Therapy may be delivered using these stimulationparameters (172). An efficacy of the stimulation may be determined(174). That efficacy may be determined using clinician observations,such as a clinician assigning a score that indicates how well thepatient is responding to therapy. An example of such a score is theUnified Parkinson's Disease Rating Scale (UPDRS) used to rate symptomsof Parkinson's disease. In some cases, automated feedback may beprovided as a user performs a task. For instance, accelerometers and/orother motion/activity sensors may be used to access tremor, determinesmoothness and/or speed of gait, the quickness of a finger tap exercise,and so on. In some case, a patient may interface with a user interface85 of a patient programmer 14 of some other user interface to obtain arating of efficacy that is partially or entirely assigned in anautomated manner. In some cases, the patient may provide feedback, as byentering information into an electronic diary.

Next, it may be determined if the parameter sweep is to continue (176).If so, one or more of the stimulation parameters may be altered andsteps 172 and 174 repeated to determine if the alternative parameter setincreased or decreases efficacy. In some cases, one parameter, such asstimulation amplitude, may be swept from a high to low value or viceversa to find a value that optimally treats patient symptoms. Powerconsideration may be taken into account when determining whether theparameter value is optimal. For instance, an amplitude that is highenough to get good patient responses, but not so high as to increaseenergy usage beyond a predetermined rate, may be selected as an optimalamount.

In the foregoing manner, any number of stimulation parameters may beswept, such as stimulation amplitude (voltage or current), pulse width,pulse rate, and burst rate (if non-regular stimulation patterns arebeing used. A “sweep” may further involve delivering a sequence ofstimulation parameters or non-pulsed waveforms to determine which of thesequence and/or waveform is most efficacious. A “sweep” may also involvedelivering stimulation using a sequence of different electrodecombinations, including bi-polar, unipolar, or multi-polarconfigurations. The foregoing activity may, in some cases, be performedwhen a fault is known to be absent in the system. In this case, theparameters that are ultimately selected as the most efficacious providea baseline set of parameters that may be stored within memory 62 and/ormemory 82. These parameters may be used to deliver therapy to a patientfor a selected period of time after the titration is performed. In somea case, after the initial sweep is considered complete (176), any faultanalysis that is performed (178) will not indicate the presence of afault.

Periodically, the method of FIG. 14 may be repeated. If a marked changein the parameters that are determined to be most efficacious occurs insteps 170-176, further fault analysis may be performed in step 178 usingany of the aforementioned approaches. For instance, if amplitude neededto deliver efficacious therapy to the patient increases above somethreshold amount or threshold percentage, it may be determined whetheran open or high-impedance condition is developing or has occurred (e.g.,due to a fraction in a lead or lead extension) in the stimulationpathway. For instance, impedance measurements may be taken to furtherconfirm whether such a condition is occurring. Additionally, oralternative, this may trigger other tests, such as the systemautomatically or with the help of a user instructing the patient toperform some motion task while measurements are being taken to determinewhether motion artifacts are now present within the sensed signal. Insome cases, measurements may be taken to determine whether cardiacartifacts are now being manifested within the signal. In some cases,stimulation may be delivered at a second location while sensing occursat a first location to determine whether the sensed signal has changedfrom the baseline signal obtained during a time when no fault waspresent in the system. In examples, the stimulation may then bedelivered at the first location while sensing is performed at a secondlocation to further pinpoint the location of any fault. In this manner,various types of trouble-shooting may occur in an automated orsemi-automated manner. Furthermore, testing involving use of sensedsignals and artifacts may uncover faults that could not be determinedusing impedance measurements or efficacy changes alone.

FIG. 15 is a flow diagram illustrating an example method according tothe current disclosure that uses various measurements within the systemto diagnose a system fault. It may be determined whether an impedancemeasurement in one or more system pathways is out of range (190). Insome example, further analysis is performed only if the impedance is outof range (192) as shown in FIG. 15. In other examples, even if impedanceis out of range, analysis may continue. This alternative approach may bedesirable to detect faults that are not necessarily detectible solely byimpedance measurements.

If further analysis is desired, low frequency stimulation may bedelivered (e.g., at a second stimulation site) and evoked potentialsignals may be detected (e.g., at a first stimulation site) as shown atstep 194. Other signals by further be introduced (e.g., at a secondsite) and sensing may be conducted (e.g., at a first site), as shown atstep 196. Such signals may be, for instance, motion signals introducedby a patient performing a specific motion or task such as turning his orher head. The introduced signals may be the beating of the patient'sheart. Other signals introduced into the patient's body may be used inaddition to or instead of the aforementioned signals, such as tapping onthe “can” of the implantable device. Additional analysis may includeperforming a parameter sweep to determine whether the efficacy oftherapy is changing over time, as may help determine whether a low orhigh impedance pathway is developing in the system (198). Any one ormore of the measurements and/or sensed signals may be used to performfault analysis.

FIG. 16 is an example dashboard screen that may be presented to a userto indicate the result of the type of testing discussed herein.Stimulation parameters are shown along the upper left column along withthe electrode configuration in use at the time. An impedance measurementprovided in the center top pattern shows impedance (in ohms) as measuredin the stimulation pathway, including a trend diagram illustrating anyimpedance changes over the last sixty days.

Below the impedance measurement data is shown a sensed brain signal (inmicrovolts) that is obtained at a first site when stimulation isdelivered to a second site, as well as trend information shown over thepast sixty days. Under this information is provided data showing batteryhealth as a percentage of full capacity, and further indicates theremaining time until next recharge is required, which in this case is 2days. Trend information is shown for the battery.

At the bottom of the central panel, symptom information is depictedusing a rating scale from “1” to “5”, with a rating of “5” indicatingsymptoms that are most severe. A UPDRS is an example of such a score. Atrend diagram illustrates how the score has changed over the past sixtydays. This score may be provided when the patient is receiving therapyusing one or more baseline therapy sets, as discussed above. Theinformation shown in FIG. 16 indicates the system is “OK” and likelyfault-free.

FIG. 17 is another example of a dashboard that may be provided to reportthe type of information described herein. The left-most panel showsparameters in use at the time of the data collection. The central panelindicates the results of brain sensing between various electrodecombinations using the displayed therapy parameters. In particular, theamplitude of the sensed LFP signal is measured, in microvolts, forstimulation delivered at various frequencies. Trend data is furtherprovided for the various electrode combinations. This data may helpdetermine if the expected baseline LFP signal is changing for one ormore electrode combinations over time, which may help determine whethera fault is developing, and if so, where in the system that fault islikely to be occurring. On the lower portion of the central panel arepatient goals, which provide symptom levels that are those set as goalsfor the patient's therapy. If the patient's symptoms are at, or below,these goals (as indicated by the dashed line for a particular symptom),good efficacy is being achieved. This goal information may be used toaccess whether efficacy is changing, as may be the case if a fault isdeveloping or has developed within the system.

FIG. 18 is an illustration of yet another dashboard that providesstimulation parameters in use to deliver therapy along the left-handside of the screen. The central panel provides summary information alongthe top. Below the summary information are changes in amplitude overtime for the stimulation that is being used to deliver therapy to thepatient. An impedance plot of impedance measured within the systembetween various electrode combinations is shown over the same period oftime. Brain sensing information illustrates brain signals (typicallyexpressed in microvolts and/or power (dB)) sensed between variouselectrode combinations over the same period of time. By plotting thisinformation over a same period of time, a user may compare trends inimpedance measurements, sensing data, and stimulation amplitude todetermine whether a fault is developing in a given stimulation orsensing pathway.

FIG. 19 is yet another example of a dashboard that may be provided to apatient to report a type of data discussed herein. The type of datashown in this diagram includes patient accelerometer data shown in themiddle of the central panel. As discussed above, such data may be usedto automatically or semi-automatically determine efficacy of therapy andtrends associated with patient symptoms. The accelerometer data includesa plot of accelerometer readings obtained by accelerometers located atvarious points on the patient's body including the left wrist, rightwrist, left ankle, and right ankle (shown along the bottom axis of theplot in the lower-right hand corner of the central panel.) Thisinformation is graphed against frequency content of the sensed signalshown along the y-axis.

The techniques described in this disclosure, including those attributedto programmer 14, IMD 16, or various constituent components, may beimplemented, at least in part, in hardware, software, firmware or anycombination thereof. For example, various aspects of the techniques maybe implemented within one or more processors, including one or moremicroprocessors, DSPs, ASICs, FPGAs, or any other equivalent integratedor discrete logic circuitry, as well as any combinations of suchcomponents, embodied in programmers, such as physician or patientprogrammers, stimulators, image processing devices or other devices. Theterm “processor” or “processing circuitry” may generally refer to any ofthe foregoing logic circuitry, alone or in combination with other logiccircuitry, or any other equivalent circuitry.

Such hardware, software, firmware may be implemented within the samedevice or within separate devices to support the various operations andfunctions described in this disclosure. While the techniques describedherein are primarily described as being performed by processor 60 of IMD16 and/or processor 80 of programmer 14, any one or more parts of thetechniques described herein may be implemented by a processor of one ofIMD 16, programmer 14, or another computing device, alone or incombination with each other. In addition, any of the described units,modules or components may be implemented together or separately asdiscrete but interoperable logic devices. Depiction of differentfeatures as modules or units is intended to highlight differentfunctional aspects and does not necessarily imply that such modules orunits must be realized by separate hardware or software components.Rather, functionality associated with one or more modules or units maybe performed by separate hardware or software components, or integratedwithin common or separate hardware or software components.

When implemented in software, the functionality ascribed to the systems,devices and techniques described in this disclosure may be embodied asinstructions on a computer-readable medium such as RAM, ROM, NVRAM,EEPROM, FLASH memory, magnetic data storage media, optical data storagemedia, or the like. The instructions may be executed to support one ormore aspects of the functionality described in this disclosure.

To reiterate, acutely-measured impedance data is currently the onlyavailable information regarding an implanted DBS system (device, leadand lead extension) and its interaction with the tissue. Impedance datahave been used to detect hardware malfunctions such as breaks or shortsin the DBS system. Whereas, several researchers have reported detectingunusually low or high impedance values associated with verified hardwarecomplications, others have reported detecting abnormal impedance valueswith no verifiable hardware problems or relatively normal impedancevalues despite verified hardware complications. In the latter set ofcases, a loss of DBS therapy benefit may result from an undetectedelectrical shunt. There is a strong need to better identify and diagnoseDBS system connection integrity issues for gauging the impact on therapyand suggest potential solutions.

In the embodiments set forth herein, a new concept is described in whichindicators of compromised DBS leads (e.g. short circuits or electricalshunts) are derived, based on brain sensing from one or more DBS leads.DBS lead integrity is evaluated using an assessment technique involvinga combination of: (1) impedance values, (2) evoked potentialmeasurements (EPs), and, (3) identification of movement or ECG artifactdetected in the sensed brain signals. A DBS lead is determined to befunctioning or non-functioning based on an aggregate outcome ofmeasures. The invention has an advantage over traditional methods, inthat it can reveal hidden failures that are masked, which result inimpedance values that appear normal despite the DBS lead integrity beingcompromised.

In one embodiment, epilepsy patients are implanted with DBS leadspositioned within the anterior nucleus (therapy) and hippocampus(seizure zone). Impedance measurements of the AN and HC DBS leads aretaken, and determined to be in or out of range. Two additionalassessments, based on signal processing of LFP signals, are performed tofurther evaluate the lead and allow for a more refined and accurateassessment. According to some examples, first, a low frequency (e.g.,2-10 Hz) stimulation from the AN DBS therapy lead is conducted, and adetermination is made to identify evoked potentials (EP) in LFP signalsof the hippocampus. A further step involves comparing the observed EP(see FIG. 9A) to a reference EP (see FIG. 9B), which may be arepresentative EP (i.e., template, with no prior data) or a sample EP orseries of EPs from the patient, which were collected when the lead wasknown to be functioning. An EP metric is shown in a system log.

According to some examples, second, LFP signals from the DBS leads areevaluated for movement or ECG artifact. Such artifacts will occur whenthe patient performs a motion task, that flexes the lead extension orlead body (including palpation at the IPG site). Motion or ECG artifactsmay also be present via leakage pathways, due to breaks or incompletesealing/insulation. The assessment for such artifacts in the brainsignals may occur during the following INS system routines: montagesweep for guided programming; real-time streaming to assess signalquality; and, background recording during baseline and/or stimulationtrials. An example is shown in FIG. 13, in which a movement artifact isobserved during real-time streaming. A movement or ECG artifact metricis indicated in a system log. In FIG. 13. LFP recordings are collectedduring real-time streaming, from both the anterior nucleus (top) andhippocampus (bottom). Note the movement artifact in the top channel,which occurred when the subject intermittently moved its head.

A similar phenomenon has also been observed in other brain structures,including the sub-thalamic nucleus (see FIGS. 11A and 11B). Enablingsensing from the STN lead indicated the DBS lead had intermittent breaksin continuity, where these occurred during movement. In FIGS. 11A and11B, there is a DBS STN lead with apparent intermittent leakage: As thesubject moves its head, a break is created that impacts stimulationrejection. This occurred in DBS lead contact pairs with apparentlynormal impedances.

A non-functioning or compromised lead is indicated by observing ordetecting (via automated template matching algorithms) a degraded evokedresponse, lack of an evoked response, or significant movement or ECGartifact. This can occur in the presence of impedance values that appearto be normal, which in isolation, would indicate the system isfunctioning properly. A consequence for this is that a patient would notbe receiving stimulation therapy when intended. Lead integrity metricsare shown in a system log for each parameter set that is tested, and maybe combined with additional biomarkers for therapy improvement.

The proposed paradigm represents a strategy in which layering occurs in“active monitoring”. The montage sweep is passive, and either evokedpotentials or a parameter sweep may prove useful as an algorithm forprogramming assistance. For example, a logical sequence first sweepsthrough frequencies, identifies one, then with that set sweeps throughamplitudes, then looks for cycling opportunity. This could be used forepilepsy and maybe dystonia/MvD/memory.

These are summarized below:

Lead Failure Metrics:

Impedance summary (out of range R values or abnormal trending of Rvalues indicating short or open circuit).

Evoked potentials in LFP signals (in the presence of out of range Ivalues or abnormal trending of I values).

Movement ECG or other electrical artifact in LFP signals (in thepresence of the abnormal I and/or R values above).

-   -   In some cases, a coincident change in therapeutic window        (therapeutic benefits relative to side effect profile)

Safety Metrics:

Presence of after-discharge (AD) detected with stimulation; AD duration.

-   -   Automatic AD shut-off.

Therapy Effectiveness Metrics:

LFP suppression index; max suppression (% rms) and duration.

Evoked potential suppression; max suppression (peak amplitude change);area under curve.

-   -   After-discharge suppression; (peak and duration).

In a second embodiment, the hippocampal lead may be considered the“therapy lead”, and as such, the assessments may be reversed. In a thirdembodiment, dual lead configurations for other therapeutic areas,including Parkinson's disease, may be used. For example, STN may be usedas a therapy site, and motor cortex may be used as a recording site. Ina fourth embodiment, single lead configurations may be assessed, withthe invention using the DBS from the “other hemisphere” as a referenceelectrode.

Various examples have been described. These and other examples arewithin the scope of the following claims.

What is claimed is:
 1. A method of detecting a fault in an implantablemedical device system, comprising: sensing a first signal via anelectrode at a first location of an anatomy of a patient; associating aportion of the first signal with a second signal introduced at a secondlocation of the anatomy of the patient; and determining whether a faultexists in the system based on one or more characteristics of theassociated portion of the first signal, wherein one or more ofassociating a portion of the first signal with the second signalintroduced at the second location in the anatomy of the patient anddetermining whether the fault exists in the system are performed by oneor more processors.
 2. The method of claim 1, wherein the first signalis a local field potential signal.
 3. The method of claim 1, wherein thefirst signal is a signal sensed from the brain of a patient.
 4. Themethod of claim 1, wherein the fault is a fault in one or more of a leador a lead extension of the implantable medical device system.
 5. Themethod of claim 1, wherein determining whether a fault exists in thesystem based on one or more characteristics of the associated portion ofthe first signal comprises detecting that a lead of the implantablemedical device system has a short circuit.
 6. The method of claim 1,wherein the second signal is stimulation delivered to the secondlocation; and wherein sensing, via an electrode at a first location ofan anatomy of a patient, comprises sensing a response to the stimulationdelivered at the second location.
 7. The method of claim 6, wherein thestimulation delivered at the second location comprises stimulationdelivered at a frequency range of between 2-10 Hz.
 8. The method ofclaim 6, wherein the first location is a hippocampus of the brain andthe second location is an anterior nucleus.
 9. The method of claim 6,wherein sensing a first signal via an electrode at a first location ofan anatomy of the patient comprises sensing a local field potential(LFP) signal via an implantable electrode at the first location of theanatomy of the patient.
 10. The method of claim 9, wherein determiningwhether a fault exists in the system based on one or morecharacteristics of the associated portion of the first signal comprisesidentifying an evoked response in the LFP signal and further comprisingcomparing the identified evoked response in the sensed LFP signal to oneor more baseline evoked response signals.
 11. The method of claim 10,wherein the one or more baseline evoked response signals arerepresentative of evoked response signals that are obtained when it isknown that a fault is not present in the system.
 12. The method of claim6, wherein the first location is a location within a first hemisphere ofthe brain and the second location is a location within a secondhemisphere of the brain.
 13. The method of claim 1, wherein the secondsignal comprises an artifact resulting from activity of the patient'sheart, and wherein determining whether a fault exists in the systembased on one or more characteristics of the associated portion of thefirst signal comprises detecting one or more characteristics of theartifact in the sensed first signal.
 14. The method of claim 13, whereinthe artifact is an ECG artifact and the sensed first signal is a brainsignal.
 15. The method of claim 1, wherein the second signal comprises amovement artifact and wherein determining whether a fault exists in thesystem based on one or more characteristics of the associated portion ofthe first signal comprises detecting one or more characteristics of themovement artifact in the sensed first signal.
 16. The method of claim15, wherein the first signal is a brain signal.
 17. The method of claim1, further comprising: determining whether one or more impedance valueswithin the implantable medical device system are out of range; and ifone or more impedance values within the implantable medical devicesystem are out of range, initiating the sensing of the first signal viathe electrode at the first location of the anatomy of a patient, theassociating of the portion of the first signal with the second signalintroduced at the second location of the anatomy of the patient and thedetermining whether the fault exists in the system based on one or morecharacteristics of the associated portion of the first signal.
 18. Asystem, comprising: a sensor configured to sense a first signal at afirst location of an anatomy of a patient; and one or more processorsconfigured to associate a portion of the first signal with a secondsignal introduced at a second location of the anatomy of the patient andto determine whether a fault exists in the system based on one or morecharacteristics of the associated portion of the first signal.
 19. Thesystem of claim 18, wherein the sensor is an electrode and the firstsignal is a local field potential signal sensed in the brain of thepatient.
 20. The system of claim 18, wherein the sensor is an electrode,and wherein the system comprises a lead carrying the electrode, andwherein the one or more processors are configured to determine whetherthe fault exists in the lead.
 21. The system of claim 18, wherein theone or more processors are configured to detect that one or a lead or alead extension of the implantable medical device system is associatedwith a low impedance pathway based on one or more characteristics of theassociated portion of the first signal.
 22. The system of claim 18,wherein the second signal is stimulation delivered to the secondlocation; and wherein the sensor is configured to sense a response tothe stimulation delivered at the second location.
 23. The system ofclaim 22, wherein the stimulation delivered at the second locationcomprises stimulation delivered at a frequency range of between 2-10 Hz.24. The system of claim 22, wherein the first location is a hippocampusof the brain and the second location is an anterior nucleus.
 25. Thesystem of claim 22, wherein the sensor is an implantable electrodeconfigured to sense a local field potential (LFP) signal.
 26. The systemof claim 25, wherein the one or more processors are configured to:determine whether the fault exists in the system based on one or morecharacteristics of the associated portion of the first signal byidentifying an evoked response in the LFP signal and compare theidentified evoked response in the sensed LFP signal to one or morebaseline evoked response signals.
 27. The system of claim 26, furthercomprising a memory to store the one or more baseline evoked responsesignals.
 28. The system of claim 18, wherein the second signal comprisesan artifact resulting from beating of the patient's heart, and whereinthe one or more processors are configured to determine whether the faultexists in the system based on one or more characteristics of theassociated portion of the first signal by detecting one or morecharacteristics of the artifact in the sensed first signal.
 29. Thesystem of claim 28, wherein the artifact is an ECG artifact and thesensed first signal is a brain signal.
 30. The system of claim 23,wherein the first location is a location within a first hemisphere ofthe brain and the second location is a location within a secondhemisphere of the brain.
 31. The system of claim 18, wherein the one ormore processors are further configured to: receive one or more valuesindicative of impedance associated with one or more components of theimplantable medical device system; and determine whether any of thereceived values are out of range; and if any of the received values areout of range: initiate sensing, via the sensor, of the first signal atthe first location of the anatomy of a patient; associate the portion ofthe first signal with a second signal introduced at a second location ofthe anatomy of the patient; and determine whether the fault exists inthe system based on one or more characteristics of the associatedportion of the first signal.